path: root/doc/groff.txt

GNU 'troff'
1 Introduction
  1.1 Background
  1.2 What Is 'groff'?
  1.3 'groff' Capabilities
  1.4 Macro Packages
  1.5 Preprocessors
  1.6 Output Devices
  1.7 Installation
  1.8 Conventions Used in This Manual
  1.9 Credits
2 Invoking 'groff'
  2.1 Options
  2.2 Environment
  2.3 Macro Directories
  2.4 Font Directories
  2.5 Paper Format
  2.6 Invocation Examples
3 Tutorial for Macro Users
  3.1 Basics
  3.2 Common Features
    3.2.1 Paragraphs
    3.2.2 Sections and Chapters
    3.2.3 Headers and Footers
    3.2.4 Page Layout
    3.2.5 Displays and Keeps
    3.2.6 Footnotes and Endnotes
    3.2.7 Table of Contents
    3.2.8 Indexing
    3.2.9 Document Formats
    3.2.10 Columnation
    3.2.11 Font and Size Changes
    3.2.12 Predefined Text
    3.2.13 Preprocessor Support
    3.2.14 Configuration and Customization
4 Macro Packages
  4.1 'man'
    4.1.1 Optional 'man' extensions
      Custom headers and footers
      Ultrix-specific man macros
      Simple example
  4.2 'mdoc'
  4.3 'me'
  4.4 'mm'
  4.5 'mom'
  4.6 'ms'
    4.6.1 Introduction
      4.6.1.1 Basic information
    4.6.2 Document Structure
    4.6.3 Document Control Settings
      Margin settings
      Titles (headers, footers)
      Text settings
      Paragraph settings
      Heading settings
      Footnote settings
      Display settings
      Other settings
    4.6.4 Document Description Macros
    4.6.5 Body Text
      4.6.5.1 Text settings
      4.6.5.2 Typographical symbols
      4.6.5.3 Paragraphs
      4.6.5.4 Headings
      4.6.5.5 Typeface and decoration
      4.6.5.6 Lists
      4.6.5.7 Indented regions
      4.6.5.8 Keeps, boxed keeps, and displays
      4.6.5.9 Tables, figures, equations, and references
      4.6.5.10 Footnotes
      4.6.5.11 Language and localization
    4.6.6 Page layout
      4.6.6.1 Headers and footers
      4.6.6.2 Tab stops
      4.6.6.3 Margins
      4.6.6.4 Multiple columns
      4.6.6.5 Creating a table of contents
    4.6.7 Differences from AT&T 'ms'
      4.6.7.1 Unix Version 7 'ms' macros not implemented by 'groff' 'ms'
    4.6.8 Legacy Features
      AT&T accent mark strings
      Berkeley accent mark and glyph strings
    4.6.9 Naming Conventions
5 GNU 'troff' Reference
  5.1 Text
    5.1.1 Filling
    5.1.2 Sentences
    5.1.3 Hyphenation
    5.1.4 Breaking
    5.1.5 Adjustment
    5.1.6 Tabs and Leaders
    5.1.7 Requests and Macros
    5.1.8 Macro Packages
    5.1.9 Input Encodings
    5.1.10 Input Conventions
  5.2 Page Geometry
  5.3 Measurements
    5.3.1 Motion Quanta
    5.3.2 Default Units
  5.4 Numeric Expressions
  5.5 Identifiers
  5.6 Formatter Instructions
    5.6.1 Control Characters
    5.6.2 Invoking Requests
    5.6.3 Calling Macros
    5.6.4 Using Escape Sequences
    5.6.5 Delimiters
  5.7 Comments
  5.8 Registers
    5.8.1 Setting Registers
    5.8.2 Interpolating Registers
    5.8.3 Auto-increment
    5.8.4 Assigning Register Formats
    5.8.5 Built-in Registers
  5.9 Manipulating Filling and Adjustment
  5.10 Manipulating Hyphenation
  5.11 Manipulating Spacing
  5.12 Tabs and Fields
    5.12.1 Leaders
    5.12.2 Fields
  5.13 Character Translations
  5.14 'troff' and 'nroff' Modes
  5.15 Line Layout
  5.16 Line Continuation
  5.17 Page Layout
  5.18 Page Control
  5.19 Using Fonts
    5.19.1 Selecting Fonts
    5.19.2 Font Families
    5.19.3 Font Positions
    5.19.4 Using Symbols
    5.19.5 Character Classes
    5.19.6 Special Fonts
    5.19.7 Artificial Fonts
    5.19.8 Ligatures and Kerning
    5.19.9 Italic Corrections
    5.19.10 Dummy Characters
  5.20 Manipulating Type Size and Vertical Spacing
    5.20.1 Changing the Type Size
    5.20.2 Changing the Vertical Spacing
    5.20.3 Using Fractional Type Sizes
  5.21 Colors
  5.22 Strings
  5.23 Conditionals and Loops
    5.23.1 Operators in Conditionals
    5.23.2 if-then
    5.23.3 if-else
    5.23.4 Conditional Blocks
    5.23.5 while
  5.24 Writing Macros
    5.24.1 Parameters
    5.24.2 Copy Mode
  5.25 Page Motions
  5.26 Drawing Geometric Objects
  5.27 Deferring Output
  5.28 Traps
    5.28.1 Vertical Position Traps
      5.28.1.1 Page Location Traps
      5.28.1.2 The Implicit Page Trap
      5.28.1.3 Diversion Traps
    5.28.2 Input Line Traps
    5.28.3 Blank Line Traps
    5.28.4 Leading Space Traps
    5.28.5 End-of-input Traps
  5.29 Diversions
  5.30 Punning Names
  5.31 Environments
  5.32 Suppressing Output
  5.33 I/O
  5.34 Postprocessor Access
  5.35 Miscellaneous
  5.36 'gtroff' Internals
  5.37 Debugging
    5.37.1 Warnings
  5.38 Implementation Differences
    5.38.1 Safer Mode
    5.38.2 Compatibility Mode
    5.38.3 Other Differences
6 File Formats
  6.1 'gtroff' Output
    6.1.1 Language Concepts
      6.1.1.1 Separation
      6.1.1.2 Argument Units
      6.1.1.3 Document Parts
    6.1.2 Command Reference
      6.1.2.1 Comment Command
      6.1.2.2 Simple Commands
      6.1.2.3 Graphics Commands
      6.1.2.4 Device Control Commands
      6.1.2.5 Obsolete Command
    6.1.3 Intermediate Output Examples
    6.1.4 Output Language Compatibility
  6.2 Device and Font Description Files
    6.2.1 'DESC' File Format
    6.2.2 Font Description File Format
Appendix A Copying This Manual
Appendix B Request Index
Appendix C Escape Sequence Index
Appendix D Operator Index
Appendix E Register Index
Appendix F Macro Index
Appendix G String Index
Appendix H File Keyword Index
Appendix I Program and File Index
Appendix J Concept Index
GNU 'troff'
***********

This manual documents GNU 'troff' version 1.23.0.

   Copyright � 1994-2023 Free Software Foundation, Inc.

     Permission is granted to copy, distribute and/or modify this
     document under the terms of the GNU Free Documentation License,
     Version 1.3 or any later version published by the Free Software
     Foundation; with no Invariant Sections, no Front-Cover Texts, and
     no Back-Cover Texts.  A copy of the license is included in the
     section entitled "GNU Free Documentation License".

1 Introduction
**************

GNU 'roff' (or 'groff') is a programming system for typesetting
documents.  It is highly flexible and has been used extensively for over
thirty years.

1.1 Background
==============

M. Douglas McIlroy, formerly of AT&T Bell Laboratories and present at
the creation of the Unix operating system, offers an authoritative
historical summary.

     The prime reason for Unix was the desire of Ken [Thompson], Dennis
     [Ritchie], and Joe Ossanna to have a pleasant environment for
     software development.  The fig leaf that got the nod from ...
     management was that an early use would be to develop a
     "stand-alone" word-processing system for use in typing pools and
     secretarial offices.  Perhaps they had in mind "dedicated", as
     distinct from "stand-alone"; that's what eventuated in various
     cases, most notably in the legal/patent department and in the AT&T
     CEO's office.

     Both those systems were targets of opportunity, not foreseen from
     the start.  When Unix was up and running on the PDP-11, Joe got
     wind of the legal department having installed a commercial word
     processor.  He went to pitch Unix as an alternative and clinched a
     trial by promising to make 'roff' able to number lines by tomorrow
     in order to fulfill a patent-office requirement that the commercial
     system did not support.

     Modems were installed so legal-department secretaries could try the
     Research machine.  They liked it and Joe's superb customer service.
     Soon the legal department got a system of their own.  Joe went on
     to create 'nroff' and 'troff'.  Document preparation became a
     widespread use of Unix, but no stand-alone word-processing system
     was ever undertaken.

   A history relating 'groff' to its predecessors 'roff', 'nroff', and
'troff' is available in the 'roff(7)' man page.

1.2 What Is 'groff'?
====================

'groff' (GNU 'roff') is a typesetting system that reads plain text input
files that include formatting commands to produce output in PostScript,
PDF, HTML, DVI, or other formats, or for display to a terminal.
Formatting commands can be low-level typesetting primitives, macros from
a supplied package, or user-defined macros.  All three approaches can be
combined.

   A reimplementation and extension of the typesetter from AT&T Unix,
'groff' is present on most POSIX systems owing to its long association
with Unix manuals (including man pages).  It and its predecessor are
notable for their production of several best-selling software
engineering texts.  'groff' is capable of producing typographically
sophisticated documents while consuming minimal system resources.

1.3 'groff' Capabilities
========================

GNU 'troff' is a typesetting document formatter; it provides a wide
range of low-level text and page operations within the framework of a
programming language.  These operations compose to generate footnotes,
tables of contents, mathematical equations, diagrams, multi-column text,
and other elements of typeset works.  Here is a survey of formatter
features; all are under precise user control.

   * text filling, breaking, alignment to the left or right margin;
     centering

   * adjustment of inter-word space size to justify text, and of
     inter-sentence space size to suit local style conventions

   * automatic and manual determination of hyphenation break points

   * pagination

   * selection of any font available to the output device

   * adjustment of type size and vertical spacing (or "leading")

   * configuration of line length and indentation amounts; columnation

   * drawing of geometric primitives (lines, arcs, polygons, circles,
     ...)

   * setup of stroke and fill colors (where supported by the output
     device)

   * embedding of hyperlinks, images, document metadata, and other
     inclusions (where supported by the output device)

1.4 Macro Packages
==================

Elemental typesetting functions can be be challenging to use directly
with complex documents.  A "macro" facility specifies how certain
routine operations, such as starting paragraphs, or printing headers and
footers, should be performed in terms of those low-level instructions.
Macros can be specific to one document or collected together into a
"macro package" for use by many.  Several macro packages available; the
most widely used are provided with 'groff'.  They are 'man', 'mdoc',
'me', 'mm', 'mom', and 'ms'.

1.5 Preprocessors
=================

An alternative approach to complexity management, particularly when
constructing tables, setting mathematics, or drawing diagrams, lies in
preprocessing.  A "preprocessor" employs a domian-specific language to
ease the generation of tables, equations, and so forth in terms that are
convenient for human entry.  Each preprocessor reads a document and
translates the parts of it that apply to it into GNU 'troff' input.
Command-line options to 'groff' tell it which preprocessors to use.

   'groff' provides preprocessors for laying out tables ('gtbl'),
typesetting equations ('geqn'), drawing diagrams ('gpic' and 'ggrn'),
inserting bibliographic references ('grefer'), and drawing chemical
structures ('gchem').  An associated program that is useful when dealing
with preprocessors is 'gsoelim'.(1)  (*note Preprocessor
Intro-Footnote-1::)

   'groff' also supports 'grap', a preprocessor for drawing graphs.  A
free implementation of it can be obtained separately.

   Unique to 'groff' is the 'preconv' preprocessor that enables 'groff'
to handle documents in a variety of input encodings.

   Other preprocessors exist, but no free implementations are known.  An
example is 'ideal', which draws diagrams using a mathematical constraint
language.

   (1) The 'g' prefix is not used on all systems; see *note Invoking
groff::.

1.6 Output Devices
==================

GNU 'troff''s output is in a device-independent page description
language, which is then read by an "output driver" that translates this
language into a file format or byte stream that a piece of (possibly
emulated) hardware understands.  'groff' features output drivers for
PostScript devices, terminal emulators (and other simple typewriter-like
machines), X11 (for previewing), TeX DVI, HP LaserJet 4/PCL5 and Canon
LBP printers (which use CaPSL), HTML, XHTML, and PDF.

1.7 Installation
================

Locate installation instructions in the files 'INSTALL',
'INSTALL.extra', and 'INSTALL.REPO' in the 'groff' source distribution.
Being a GNU project, 'groff' supports the familiar './configure && make'
command sequence.

1.8 Conventions Used in This Manual
===================================

We apply the term "groff" to the language documented here, the GNU
implementation of the overall system, the project that develops that
system, and the command of that name.  In the first sense, 'groff' is an
extended dialect of the 'roff' language, for which many similar
implementations exist.

   The 'roff' language features several major categories for which many
items are predefined.  Presentations of these items feature the form in
which the item is most commonly used on the left, and, aligned to the
right margin, the name of the category in brackets.

 -- Register: \n[example]
     The register 'example' is one that that 'groff' _doesn't_
     predefine.  You can create it yourself, though; see *note Setting
     Registers::.

   To make this document useful as a reference and not merely amiable
bedtime reading, we tend to present these syntax items in exhaustive
detail when they arise.  References to topics discussed later in the
text are frequent; skip material you don't understand yet.

   We use Texinfo's "result" (=>) and error-> notations to present
output written to the standard output and standard error streams,
respectively.  Diagnostic messages from the GNU 'troff' formatter and
other programs are examples of the latter, but the formatter can also be
directed to write user-specified messages to the standard error stream.
The notation then serves to identify the output stream and does not
necessarily mean that an error has occurred.(1)  (*note Conventions Used
in This Manual-Footnote-1::)

     $ echo "Twelve o'clock and" | groff -Tascii | sed '/^$/d'
         => Twelve o'clock and
     $ echo '.tm all is well.' | groff > /dev/null
         error-> all is well.

   Sometimes we use => somewhat abstractly to represent formatted text
that you will need to use a PostScript or PDF viewer program (or a
printer) to observe.  While arguably an abuse of notation, we think this
preferable to requiring the reader to understand the syntax of these
page description languages.

   We also present diagnostic messages in an abbreviated form, often
omitting the name of the program issuing them, the input file name, and
line number or other positional information when such data do not serve
to illuminate the topic under discussion.

   Most examples are of 'roff' language input that would be placed in a
text file.  Occasionally, we start an example with a '$' character to
indicate a shell prompt, as seen above.

   You are encouraged to try the examples yourself, and to alter them to
better learn 'groff''s behavior.  Our examples frequently need to direct
the formatter to set a line length (with '.ll') that will fit within the
page margins of this manual.  We mention this so that you know why it is
there before we discuss the 'll' request formally.(2)  (*note
Conventions Used in This Manual-Footnote-2::)

   (1) Unix and related operating systems distinguish standard output
and standard error streams _because_ of 'troff':
<https://minnie.tuhs.org/pipermail/tuhs/2013-December/006113.html>.

   (2) *Note Line Layout::.

1.9 Credits
===========

We adapted portions of this manual from existing documents.  James
Clark's man pages were an essential resource; we have updated them in
parallel with the development of this manual.  We based the tutorial for
macro users on Eric Allman's introduction to his 'me' macro package
(which we also provide, little altered from 4.4BSD). Larry Kollar
contributed much of the material on the 'ms' macro package.

2 Invoking 'groff'
******************

This chapter focuses on how to invoke the 'groff' front end.  This front
end takes care of the details of constructing the pipeline among the
preprocessors, 'gtroff' and the postprocessor.

   It has become a tradition that GNU programs get the prefix 'g' to
distinguish them from their original counterparts provided by the host
(*note Environment::).  Thus, for example, 'geqn' is GNU 'eqn'.  On
operating systems like GNU/Linux or the Hurd, which don't contain
proprietary versions of 'troff', and on MS-DOS/MS-Windows, where 'troff'
and associated programs are not available at all, this prefix is omitted
since GNU 'troff' is the only incarnation of 'troff' used.  Exception:
'groff' is never replaced by 'roff'.

   In this document, we consequently say 'gtroff' when talking about the
GNU 'troff' program.  All other implementations of 'troff' are called
AT&T 'troff', which is the common origin of almost all 'troff'
implementations(1) (*note Invoking groff-Footnote-1::) (with more or
less compatible changes).  Similarly, we say 'gpic', 'geqn', and so on.

   (1) Besides 'groff', 'neatroff' is an exception.

2.1 Options
===========

'groff' normally runs the 'gtroff' program and a postprocessor
appropriate for the selected device.  The default device is 'ps' (but it
can be changed when 'groff' is configured and built).  It can optionally
preprocess with any of 'gpic', 'geqn', 'gtbl', 'ggrn', 'grap', 'gchem',
'grefer', 'gsoelim', or 'preconv'.

   This section documents only options to the 'groff' front end.  Many
of the arguments to 'groff' are passed on to 'gtroff'; therefore, those
are also included.  Arguments to preprocessors and output drivers can be
found in the man pages 'gpic(1)', 'geqn(1)', 'gtbl(1)', 'ggrn(1)',
'grefer(1)', 'gchem(1)', 'gsoelim(1)', 'preconv(1)', 'grotty(1)',
'grops(1)', 'gropdf(1)', 'grohtml(1)', 'grodvi(1)', 'grolj4(1)',
'grolbp(1)', and 'gxditview(1)'.

   The command-line format for 'groff' is:

     groff [ -abceghijklpstvzCEGNRSUVXZ ] [ -dCS ] [ -DARG ]
           [ -fFAM ] [ -FDIR ] [ -IDIR ] [ -KARG ]
           [ -LARG ] [ -mNAME ] [ -MDIR ] [ -nNUM ]
           [ -oLIST ] [ -PARG ] [ -rCN ] [ -TDEV ]
           [ -wNAME ] [ -WNAME ] [ FILES... ]

   The command-line format for 'gtroff' is as follows.

     gtroff [ -abcivzCERU ] [ -dCS ] [ -fFAM ] [ -FDIR ]
            [ -mNAME ] [ -MDIR ] [ -nNUM ] [ -oLIST ]
            [ -rCN ] [ -TNAME ] [ -wNAME ] [ -WNAME ]
            [ FILES... ]

Obviously, many of the options to 'groff' are actually passed on to
'gtroff'.

   Options without an argument can be grouped behind a single '-'.  A
filename of '-' denotes the standard input.  Whitespace is permitted
between an option and its argument.

   The 'grog' command can be used to guess the correct 'groff' command
to format a file.  See its man page 'grog(1)'; type 'man grog' at the
command line to view it.

   'groff''s command-line options are as follows.

'-a'
     Generate a plain text approximation of the typeset output.  The
     read-only register '.A' is set to 1.  *Note Built-in Registers::.
     This option produces a sort of abstract preview of the formatted
     output.

        * Page breaks are marked by a phrase in angle brackets; for
          example, '<beginning of page>'.

        * Lines are broken where they would be in the formatted output.

        * A horizontal motion of any size is represented as one space.
          Adjacent horizontal motions are not combined.  Inter-sentence
          space nodes (those arising from the second argument to the
          'ss' request) are not represented.

        * Vertical motions are not represented.

        * Special characters are rendered in angle brackets; for
          example, the default soft hyphen character appears as '<hy>'.

     The above description should not be considered a specification; the
     details of '-a' output are subject to change.

'-b'
     Write a backtrace reporting the state of 'gtroff''s input parser to
     the standard error stream with each diagnostic message.  The line
     numbers given in the backtrace might not always be correct, because
     'gtroff''s idea of line numbers can be confused by requests that
     append to macros.

'-c'
     Start with color output disabled.

'-C'
     Enable AT&T 'troff' compatibility mode; implies '-c'.  *Note
     Implementation Differences::, for the list of incompatibilities
     between 'groff' and AT&T 'troff'.

'-dCTEXT'
'-dSTRING=TEXT'
     Define 'roff' string C or STRING as T or TEXT.  C must be one
     character; STRING can be of arbitrary length.  Such string
     assignments happen before any macro file is loaded, including the
     startup file.  Due to 'getopt_long' limitations, C cannot be, and
     STRING cannot contain, an equals sign, even though that is a valid
     character in a 'roff' identifier.

'-DENC'
     Set fallback input encoding used by 'preconv' to ENC; implies '-k'.

'-e'
     Run 'geqn' preprocessor.

'-E'
     Inhibit 'gtroff' error messages.  This option does _not_ suppress
     messages sent to the standard error stream by documents or macro
     packages using 'tm' or related requests.

'-fFAM'
     Use FAM as the default font family.  *Note Font Families::.

'-FDIR'
     Search in directory 'DIR' for the selected output device's
     directory of device and font description files.  See the
     description of 'GROFF_FONT_PATH' in *note Environment:: below for
     the default search locations and ordering.

'-g'
     Run 'ggrn' preprocessor.

'-G'
     Run 'grap' preprocessor; implies '-p'.

'-h'
     Display a usage message and exit.

'-i'
     Read the standard input after all the named input files have been
     processed.

'-IDIR'
     Search the directory DIR for files named in several contexts;
     implies '-g' and '-s'.

        * 'gsoelim' replaces 'so' requests with the contents of their
          file name arguments.

        * 'gtroff' searches for files named as operands in its command
          line and as arguments to 'psbb', 'so', and 'soquiet' requests.

        * Output drivers may search for files; for instance, 'grops'
          looks for files named in '\X'ps: import ...'', '\X'ps: file
          ...'', and '\X'pdf: pdfpic ...'' device control escape
          sequences.

     This option may be specified more than once; the directories are
     searched in the order specified.  If you want to search the current
     directory before others, add '-I .' at the desired place.  The
     current working directory is otherwise searched last.  '-I' works
     similarly to, and is named for, the "include" option of Unix C
     compilers.

     '-I' options are passed to 'gsoelim', 'gtroff', and output drivers;
     with the flag letter changed to '-M', they are also passed to
     'ggrn'.

'-j'
     Run 'gchem' preprocessor.  Implies '-p'.

'-k'
     Run 'preconv' preprocessor.  Refer to its man page for its behavior
     if neither of 'groff''s '-K' or '-D' options is also specified.

'-KENC'
     Set input encoding used by 'preconv' to ENC; implies '-k'.

'-l'
     Send the output to a spooler for printing.  The 'print' directive
     in the device description file specifies the default command to be
     used; see *note Device and Font Description Files::.  See options
     '-L' and '-X'.

'-LARG'
     Pass ARG to the print spooler program.  If multiple ARGs are
     required, pass each with a separate '-L' option.  'groff' does not
     prefix an option dash to ARG before passing it to the spooler
     program.

'-mNAME'
     Process the file 'NAME.tmac' prior to any input files.  If not
     found, 'tmac.NAME' is attempted.  NAME (in both arrangements) is
     presumed to be a macro file; see the description of
     'GROFF_TMAC_PATH' in *note Environment:: below for the default
     search locations and ordering.  This option and its argument are
     also passed to 'geqn', 'grap', and 'ggrn'.

'-MDIR'
     Search directory 'DIR' for macro files; see the description of
     'GROFF_TMAC_PATH' in *note Environment:: below for the default
     search locations and ordering.  This option and its argument are
     also passed to 'geqn', 'grap', and 'ggrn'.

'-nNUM'
     Number the first page NUM.

'-N'
     Prohibit newlines between 'eqn' delimiters: pass '-N' to 'geqn'.

'-oLIST'
     Output only pages in LIST, which is a comma-separated list of page
     ranges; 'N' means page N, 'M-N' means every page between M and N,
     '-N' means every page up to N, 'N-' means every page from N on.
     'gtroff' stops processing and exits after formatting the last page
     enumerated in LIST.

'-p'
     Run 'gpic' preprocessor.

'-PARG'
     Pass ARG to the postprocessor.  If multiple ARGs are required, pass
     each with a separate '-P' option.  'groff' does not prefix an
     option dash to ARG before passing it to the postprocessor.

'-rCNUMERIC-EXPRESSION'
'-rREGISTER=EXPR'
     Set 'roff' register C or REGISTER to the value NUMERIC-EXPRESSION
     (*note Numeric Expressions::).  C must be one character; REGISTER
     can be of arbitrary length.  Such register assignments happen
     before any macro file is loaded, including the startup file.  Due
     to 'getopt_long' limitations, C cannot be, and REGISTER cannot
     contain, an equals sign, even though that is a valid character in a
     'roff' identifier.

'-R'
     Run 'grefer' preprocessor.  No mechanism is provided for passing
     arguments to 'grefer' because most 'grefer' options have equivalent
     language elements that can be specified within the document.

     'gtroff' also accepts a '-R' option, which is not accessible via
     'groff'.  This option prevents the loading of the 'troffrc' and
     'troffrc-end' files.

'-s'
     Run 'gsoelim' preprocessor.

'-S'
     Operate in "safer" mode; see '-U' below for its opposite.  For
     security reasons, safer mode is enabled by default.

'-t'
     Run 'gtbl' preprocessor.

'-TDEV'
     Direct 'gtroff' to format the input for the output device DEV.
     'groff' then calls an output driver to convert 'gtroff''s output to
     a form appropriate for DEV.  The following output devices are
     available.

     'ps'
          For PostScript printers and previewers.

     'pdf'
          For PDF viewers or printers.

     'dvi'
          For TeX DVI format.

     'X75'
          For a 75dpi X11 previewer.

     'X75-12'
          For a 75dpi X11 previewer with a 12-point base font in the
          document.

     'X100'
          For a 100dpi X11 previewer.

     'X100-12'
          For a 100dpi X11 previewer with a 12-point base font in the
          document.

     'ascii'
          For typewriter-like devices using the (7-bit) ASCII (ISO 646)
          character set.

     'latin1'
          For typewriter-like devices that support the Latin-1
          (ISO 8859-1) character set.

     'utf8'
          For typewriter-like devices that use the Unicode (ISO 10646)
          character set with UTF-8 encoding.

     'cp1047'
          For typewriter-like devices that use the EBCDIC encoding IBM
          code page 1047.

     'lj4'
          For HP LaserJet4-compatible (or other PCL5-compatible)
          printers.

     'lbp'
          For Canon CaPSL printers (LBP-4 and LBP-8 series laser
          printers).

     'html'
     'xhtml'
          To produce HTML and XHTML output, respectively.  This driver
          consists of two parts, a preprocessor ('pre-grohtml') and a
          postprocessor ('post-grohtml').

     The predefined GNU 'troff' string '.T' contains the name of the
     output device; the read-only register '.T' is set to 1 if this
     option is used (which is always true if 'groff' is used to call GNU
     'troff').  *Note Built-in Registers::.

     The postprocessor to be used for a device is specified by the
     'postpro' command in the device description file.  (*Note Device
     and Font Description Files::.)  This can be overridden with the
     '-X' option.

'-U'
     Operate in "unsafe mode", which enables the 'open', 'opena', 'pi',
     'pso', and 'sy' requests.  These requests are disabled by default
     because they allow an untrusted input document to write to
     arbitrary file names and run arbitrary commands.  This option also
     adds the current directory to the macro package search path; see
     the '-m' option above.  '-U' is passed to 'gpic' and 'gtroff'.

'-v'
     Write version information for 'groff' and all programs run by it to
     the standard output stream; that is, the given command line is
     processed in the usual way, passing '-v' to the formatter and any
     pre- or postprocessors invoked.

'-V'
     Output the pipeline that would be run by 'groff' (as a wrapper
     program) to the standard output stream, but do not execute it.  If
     given more than once, the pipeline is both written to the standard
     error stream and run.

'-wCATEGORY'
     Enable warnings in CATEGORY.  Categories are listed in *note
     Warnings::.

'-WCATEGORY'
     Inhibit warnings in CATEGORY.  Categories are listed in *note
     Warnings::.

'-X'
     Use 'gxditview' instead of the usual postprocessor to (pre)view a
     document on an X11 display.  Combining this option with '-Tps' uses
     the font metrics of the PostScript device, whereas the '-TX75' and
     '-TX100' options use the metrics of X11 fonts.

'-z'
     Suppress formatted output from 'gtroff'.

'-Z'
     Disable postprocessing.  'gtroff' output will appear on the
     standard output stream (unless suppressed with '-z'; see *note
     gtroff Output:: for a description of this format.

2.2 Environment
===============

There are also several environment variables (of the operating system,
not within 'gtroff') that can modify the behavior of 'groff'.

'GROFF_BIN_PATH'
     This search path, followed by 'PATH', is used for commands executed
     by 'groff'.

'GROFF_COMMAND_PREFIX'
     If this is set to X, then 'groff' runs 'Xtroff' instead of
     'gtroff'.  This also applies to 'tbl', 'pic', 'eqn', 'grn', 'chem',
     'refer', and 'soelim'.  It does not apply to 'grops', 'grodvi',
     'grotty', 'pre-grohtml', 'post-grohtml', 'preconv', 'grolj4',
     'gropdf', and 'gxditview'.

     The default command prefix is determined during the installation
     process.  If a non-GNU 'troff' system is found, prefix 'g' is used,
     none otherwise.

'GROFF_ENCODING'
     The value of this variable is passed to the 'preconv'
     preprocessor's '-e' option to select the character encoding of
     input files.  This variable's existence implies the 'groff' option
     '-k'.  If set but empty, 'groff' calls 'preconv' without an '-e'
     option.  'groff''s '-K' option overrides 'GROFF_ENCODING'.  See the
     'preconv(7)' man page; type 'man preconv' at the command line to
     view it.

'GROFF_FONT_PATH'
     A list of directories in which to seek the selected output device's
     directory of device and font description files.  GNU 'troff' will
     search directories given as arguments to any specified '-F' options
     before these, and a built-in list of directories after them.  *Note
     Font Directories:: and the 'troff(1)' or 'gtroff(1)' man pages.

'GROFF_TMAC_PATH'
     A list of directories in which to seek macro files.  GNU 'troff'
     will search directories given as arguments to any specified '-M'
     options before these, and a built-in list of directories after
     them.  *Note Macro Directories:: and the 'troff(1)' or 'gtroff(1)'
     man pages.

'GROFF_TMPDIR'
     The directory in which 'groff' creates temporary files.  If this is
     not set and 'TMPDIR' is set, temporary files are created in that
     directory.  Otherwise temporary files are created in a
     system-dependent default directory (on Unix and GNU/Linux systems,
     this is usually '/tmp').  'grops', 'grefer', 'pre-grohtml', and
     'post-grohtml' can create temporary files in this directory.

'GROFF_TYPESETTER'
     Sets the default output device.  If empty or not set, a build-time
     default (often 'ps') is used.  The '-TDEV' option overrides
     'GROFF_TYPESETTER'.

'SOURCE_DATE_EPOCH'
     A timestamp (expressed as seconds since the Unix epoch) to use as
     the output creation timestamp in place of the current time.  The
     time is converted to human-readable form using 'localtime(3)' when
     the formatter starts up and stored in registers usable by documents
     and macro packages (*note Built-in Registers::).

'TZ'
     The time zone to use when converting the current time (or value of
     'SOURCE_DATE_EPOCH') to human-readable form; see 'tzset(3)'.

   MS-DOS and MS-Windows ports of 'groff' use semicolons, rather than
colons, to separate the directories in the lists described above.

2.3 Macro Directories
=====================

A macro file must have a name in the form 'NAME.tmac' or 'tmac.NAME' and
be placed in a "tmac directory" to be found by the '-mNAME' command-line
option.(1)  (*note Macro Directories-Footnote-1::) Together, these
directories constitute the "tmac path".  Each directory is searched in
the following order until the desired macro file is found or the list is
exhausted.

   * Directories specified with GNU 'troff''s or 'groff''s '-M'
     command-line option.

   * Directories listed in the 'GROFF_TMAC_PATH' environment variable.

   * The current working directory (only if in unsafe mode using the
     '-U' command-line option).

   * The user's home directory, 'HOME'.

   * A platform-dependent directory, a site-local (platform-independent)
     directory, and the main tmac directory.  The locations
     corresponding to your installation are listed in section
     "Environment" of 'gtroff(1)'.  If not otherwise configured, they
     are as follows.

          /usr/local/lib/groff/site-tmac
          /usr/local/share/groff/site-tmac
          /usr/local/share/groff/1.23.0/tmac

     The foregoing assumes that the version of 'groff' is 1.23.0, and
     that the installation prefix was '/usr/local'.  It is possible to
     fine-tune these locations during the source configuration process.

   (1) The 'mso' request does not have these limitations.  *Note I/O::.

2.4 Font Directories
====================

'groff' enforces few restrictions on how font description files are
named.  For its family/style mechanism to work (*note Font Families::),
the names of fonts within a family should start with the family name,
followed by the style.  For example, the Times family uses 'T' for the
family name and 'R', 'B', 'I', and 'BI' to indicate the styles 'roman',
'bold', 'italic', and 'bold italic', respectively.  Thus the final font
names are 'TR', 'TB', 'TI', and 'TBI'.

   Font description files are kept in "font directories", which together
constitute the "font path".  The search procedure always appends the
directory 'dev'NAME, where NAME is the name of the output device.
Assuming TeX DVI output, and '/foo/bar' as a font directory, the font
description files for 'grodvi' must be in '/foo/bar/devdvi'.  Each
directory in the font path is searched in the following order until the
desired font description file is found or the list is exhausted.

   * Directories specified with GNU 'troff''s or 'groff''s '-f'
     command-line option.  All output drivers (and some preprocessors)
     support this option as well, because they require information about
     the glyphs to be rendered in the document.

   * Directories listed in the 'GROFF_FONT_PATH' environment variable.

   * A site-local directory and the main font description directory.
     The locations corresponding to your installation are listed in
     section "Environment" of 'gtroff(1)'.  If not otherwise configured,
     they are as follows.

          /usr/local/share/groff/site-font
          /usr/local/share/groff/1.23.0/font

     The foregoing assumes that the version of 'groff' is 1.23.0, and
     that the installation prefix was '/usr/local'.  It is possible to
     fine-tune these locations during the source configuration process.

2.5 Paper Format
================

In 'groff', the page dimensions for the formatter GNU 'troff' and for
output devices are handled separately.  *Note Page Layout::, for
vertical manipulation of the page size, and *Note Line Layout::, for
horizontal changes.  The 'papersize' macro package, normally loaded by
'troffrc' at startup, provides an interface for configuring page
dimensions by convenient names, like 'letter' or 'a4'; see
'groff_tmac(5)'.  The default used by the formatter depends on its build
configuration, but is usually one of the foregoing, as geographically
appropriate.

   It is up to each macro package to respect the page dimensions
configured in this way.

   For each output device, the size of the output medium can be set in
its 'DESC' file.  Most output drivers also recognize a command-line
option '-p' to override the default dimensions and an option '-l' to use
landscape orientation.  *Note DESC File Format::, for a description of
the 'papersize' keyword, which takes an argument of the same form as
'-p'.  The output driver's man page, such as 'grops(1)', may also be
helpful.

   'groff' uses the command-line option '-P' to pass options to
postprocessors; for example, use the following for PostScript output on
A4 paper in landscape orientation.

     groff -Tps -dpaper=a4l -P-pa4 -P-l -ms foo.ms > foo.ps

2.6 Invocation Examples
=======================

'roff' systems are best known for formatting man pages.  Once a 'man'
librarian program has located a man page, it may execute a 'groff'
command much like the following.

     groff -t -man -Tutf8 /usr/share/man/man1/groff.1

   The librarian will also pipe the output through a pager, which might
not interpret the SGR terminal escape sequences 'groff' emits for
boldface, underlining, or italics; see the 'grotty(1)' man page for a
discussion.

   To process a 'roff' input file using the preprocessors 'gtbl' and
'gpic' and the 'me' macro package in the way to which AT&T 'troff' users
were accustomed, one would type (or script) a pipeline.

     gpic foo.me | gtbl | gtroff -me -Tutf8 | grotty

   Using 'groff', this pipe can be shortened to an equivalent command.

     groff -p -t -me -T utf8 foo.me

   An even easier way to do this is to use 'grog' to guess the
preprocessor and macro options and execute the result by using the
command substitution feature of the shell.

     $(grog -Tutf8 foo.me)

   Each command-line option to a postprocessor must be specified with
any required leading dashes '-' because 'groff' passes the arguments
as-is to the postprocessor; this permits arbitrary arguments to be
transmitted.  For example, to pass a title to the 'gxditview'
postprocessor, the shell commands

     groff -X -P -title -P 'trial run' mydoc.t

and

     groff -X -Z mydoc.t | gxditview -title 'trial run' -

are equivalent.

3 Tutorial for Macro Users
**************************

Most users of the 'roff' language employ a macro package to format their
documents.  Successful macro packages ease the composition process;
their users need not have mastered the full formatting language, nor
understand features like diversions, traps, and environments.  This
chapter aims to familiarize you with basic concepts and mechanisms
common to many macro packages (like "displays").  If you prefer a
meticulous and comprehensive presentation, try *note GNU troff
Reference:: instead.

3.1 Basics
==========

Let us first survey some basic concepts necessary to use a macro package
fruitfully.(1)  (*note Basics-Footnote-1::) References are made
throughout to more detailed information.

   GNU 'troff' reads an input file prepared by the user and outputs a
formatted document suitable for publication or framing.  The input
consists of text, or words to be printed, and embedded commands
(requests and escape sequences), which tell GNU 'troff' how to format
the output.  *Note Formatter Instructions::.

   The word argument is used in this chapter to mean a word or number
that appears on the same line as a request, and which modifies the
meaning of that request.  For example, the request

     .sp

spaces one line, but

     .sp 4

spaces four lines.  The number 4 is an argument to the 'sp' request,
which says to space four lines instead of one.  Arguments are separated
from the request and from each other by spaces (_not_ tabs).  *Note
Invoking Requests::.

   The primary function of GNU 'troff' is to collect words from input
lines, fill output lines with those words, adjust the line to the
right-hand margin by widening spaces, and output the result.  For
example, the input:

     Now is the time
     for all good men
     to come to the aid
     of their party.
     Four score and seven
     years ago, etc.

is read, packed onto output lines, and justified to produce:

       => Now is the time for all good men to come to the aid of
       => their party.  Four score and seven years ago, etc.

   Sometimes a new output line should be started even though the current
line is not yet full--for example, at the end of a paragraph.  To do
this it is possible to force a break, starting a new output line.  Some
requests cause a break automatically, as do (normally) blank input lines
and input lines beginning with a space or tab.

   Not all input lines are text lines--words to be formatted.  Some are
control lines that tell a macro package (or GNU 'troff' directly) how to
format the text.  Control lines start with a dot ('.') or an apostrophe
(''') as the first character, and can be followed by a macro call.

   The formatter also does more complex things, such as automatically
numbering pages, skipping over page boundaries, putting footnotes in the
correct place, and so forth.

   Here are a few hints for preparing text for input to GNU 'troff'.

   * First, keep the input lines short.  Short input lines are easier to
     edit, and GNU 'troff' packs words onto longer lines anyhow.

   * In keeping with this, it is helpful to begin a new line after every
     comma or phrase, since common corrections are to add or delete
     sentences or phrases.

   * End each sentence with two spaces--or better, start each sentence
     on a new line.  GNU 'troff' recognizes characters that usually end
     a sentence, and inserts inter-sentence space accordingly.

   * Do not hyphenate words at the end of lines--GNU 'troff' is smart
     enough to hyphenate words as needed, but is not smart enough to
     take hyphens out and join a word back together.  Also, words such
     as "mother-in-law" should not be broken over a line, since then a
     space can occur where not wanted, such as "mother- in-law".

   We offer further advice in *note Input Conventions::.

   GNU 'troff' permits alteration of the distance between lines of text.
This is termed vertical spacing and is expressed in the same units as
the type size--the point.  The default is 10-point type on 12-point
spacing.  To get double-spaced text you would set the vertical spacing
to 24 points.  Some, but not all, macro packages expose a macro or
register to configure the vertical spacing.

   A number of requests allow you to change the way the output is
arranged on the page, sometimes called the layout of the output page.
Most macro packages don't supply macros for performing these (at least
not without performing other actions besides), as they are such basic
operations.  The macro packages for writing man pages, 'man' and 'mdoc',
don't encourage explicit use of these requests at all.

   The request '.sp N' leaves N lines of blank space.  N can be omitted
(skipping a single line) or can be of the form Ni (for N inches) or Nc
(for N centimeters).  For example, the input:

     .sp 1.5i
     My thoughts on the subject
     .sp

leaves one and a half inches of space, followed by the line "My thoughts
on the subject", followed by a single blank line (more measurement units
are available; see *note Measurements::).

   If you seek precision in spacing, be advised when using a macro
package that it might not honor 'sp' requests as you expect; it can use
a formatter feature called no-space mode to prevent excess space from
accumulating.  Macro packages typically offer registers to control
spacing between paragraphs, before section headings, and around displays
(discussed below); use these facilities preferentially.  *Note
Manipulating Spacing::.

   Text lines can be centered by using the 'ce' request.  The line after
'ce' is centered (horizontally) on the page.  To center more than one
line, use '.ce N' (where N is the number of lines to center), followed
by the N lines.  To center many lines without counting them, type:

     .ce 1000
     lines to center
     .ce 0

The '.ce 0' request tells GNU 'troff' to center zero more lines, in
other words, stop centering.

   GNU 'troff' also offers the 'rj' request for right-aligning text.  It
works analogously to 'ce' and is convenient for setting epigraphs.

   The 'bp' request starts a new page; this necessarily implies an
ordinary (line) break.

   All of these requests cause a break; that is, they always start a new
line.  To start a new line without performing any other action, use
'br'.  If you invoke them with the apostrophe ''', the no-break control
character, the (initial) break they normally perform is suppressed.
''br' does nothing.

   (1) The remainder of this chapter is based on 'Writing Papers with
nroff using -me' by Eric P. Allman, which is distributed with 'groff' as
'meintro.me'.

3.2 Common Features
===================

GNU 'troff' provides low-level operations for formatting a document.
Many routine operations are undertaken in nearly all documents that
require a series of such primitive operations to be performed.  These
common tasks are grouped into macros, which are then collected into a
macro package.

   Macro packages come in two varieties: "major" or "full-service" ones
that manage page layout, and "minor" or "auxiliary" ones that do not,
instead fulfilling narrow, specific tasks.  Find a list in the
'groff_tmac(5)' man page.  Type 'man groff_tmac' at the command line to
view it.

   We survey several capabilities of full-service macro package below.
Each package employs its own macros to exercise them.  For details,
consult its man page or, for 'ms', see *note ms::.

3.2.1 Paragraphs
----------------

Paragraphs can be separated and indented in various ways.  Some start
with a blank line and have a first-line indentation, like most of the
ones in this manual.  Block paragraphs omit the indentation.

       => Some  men  look  at constitutions with sanctimonious
       => reverence,  and  deem  them  like  the  ark  of  the
       => covenant, too sacred to be touched.

We also frequently encounter tagged paragraphs, which begin with a tag
or label at the left margin and indent the remaining text.

       => one  This  is the first paragraph.  Notice how the
       =>      first line of the resulting  paragraph  lines
       =>      up with the other lines in the paragraph.

If the tag is too wide for the indentation, the line is broken.

       => longlabel
       =>      The  label does not align with the subsequent
       =>      lines, but they align with each other.

A variation of the tagged paragraph is the itemized or enumerated
paragraph, which might use punctuation or a digit for a tag,
respectively.  These are frequently used to construct lists.

       => o    This  list  item  starts with a bullet.  When
       =>      producing output for a device using the ASCII
       =>      character set, an 'o' is formatted instead.

Often, use of the same macro without a tag continues such a discussion.

       => -xyz  This option is recognized but ignored.
       =>
       =>       It had a security hole that we don't discuss.

3.2.2 Sections and Chapters
---------------------------

The simplest kind of section heading is unnumbered, set in a bold or
italic style, and occupies a line by itself.  Others possess
automatically numbered multi-level headings and/or different typeface
styles or sizes at different levels.  More sophisticated macro packages
supply macros for designating chapters and appendices.

3.2.3 Headers and Footers
-------------------------

Headers and footers occupy the top and bottom of each page,
respectively, and contain data like the page number and the article or
chapter title.  Their appearance is not affected by the running text.
Some packages allow for different titles on even- and odd-numbered pages
(for printed, bound material).

   Headers and footers are together called titles, and comprise three
parts: left-aligned, centered, and right-aligned.  A '%' character
appearing anywhere in a title is automatically replaced by the page
number.  *Note Page Layout::.

3.2.4 Page Layout
-----------------

Most macro packages let the user specify the size of the page margins.
The top and bottom margins are typically handled differently than the
left and right margins; the latter two are derived from the page offset,
indentation, and line length.  *Note Line Layout::.  Commonly, packages
support registers to tune these values.

3.2.5 Displays and Keeps
------------------------

Displays are sections of text set off from the surrounding material
(typically paragraphs), often differing in indentation, and/or spacing.
Tables, block quotations, and figures are displayed.  Equations and code
examples, when not much shorter than an output line, often are.  Lists
may or may not be.  Packages for setting man pages support example
displays but not keeps.

   A keep is a group of output lines, often a display, that is formatted
on a single page if possible; it causes a page break to happen early so
as to not interrupt the kept material.

   Floating keeps can move, or "float", relative to the text around them
in the input.  They are useful for displays that are captioned and
referred to by name, as with "See figure 3".  Depending on the package,
a floating keep appears at the bottom of the current page if it fits,
and at the top of the next otherwise.  Alternatively, floating keeps
might be deferred to the end of a section.  Using a floating keep can
avoid the large vertical spaces that may precede a tall keep of the
ordinary sort when it won't fit on the page.

3.2.6 Footnotes and Endnotes
----------------------------

Footnotes and endnotes are forms of delayed formatting.  They are
recorded at their points of relevance in the input, but not formatted
there.  Instead, a mark cues the reader to check the "foot", or bottom,
of the current page, or in the case of endnotes, an annotation list
later in the document.  Macro packages that support these features also
supply a means of automatically numbering either type of annotation.

3.2.7 Table of Contents
-----------------------

A package may handle a table of contents by directing section heading
macros to save section heading text and the page number where it occurs
for use in a later entry for a table of contents.  It writes the
collected entries at the end of the document, once all are known, upon
request.  A row of dots (a leader) bridges the text on the left with its
location on the right.  Other collections might work in this manner,
providing lists of figures or tables.

   A table of contents is often found at the end of a GNU 'troff'
document because the formatter processes the document in a single pass.
The 'gropdf' output driver supports a PDF feature that relocates pages
at the time the document is rendered; see the 'gropdf(1)' man page.
Type 'man gropdf' at the command line to view it.

3.2.8 Indexing
--------------

An index is similar to a table of contents, in that entry labels and
locations must be collected, but poses a greater challenge because it
needs to be sorted before it is output.  Here, processing the document
in multiple passes is inescapable, and tools like the 'makeindex'
program are necessary.

3.2.9 Document Formats
----------------------

Some macro packages supply stock configurations of certain documents,
like business letters and memoranda.  These often also have provision
for a cover sheet, which may be rigid in its format.  With these
features, it is even more important to use the package's macros in
preference to the formatter requests presented earlier, where possible.

3.2.10 Columnation
------------------

Macro packages apart from 'man' and 'mdoc' for man page formatting offer
a facility for setting multiple columns on the page.

3.2.11 Font and Size Changes
----------------------------

The formatter's requests and escape sequences for setting the typeface
and size are not always intuitive, so all macro packages provide macros
to make these operations simpler.  They also make it more convenient to
change typefaces in the middle of a word and can handle italic
corrections automatically.  *Note Italic Corrections::.

3.2.12 Predefined Text
----------------------

Most macro packages supply predefined strings to set prepared text like
the date, or to perform operations like super- and subscripting.

3.2.13 Preprocessor Support
---------------------------

All macro packages provide support for various preprocessors and may
extend their functionality by defining macros to set their contents in
displays.  Examples include 'TS' and 'TE' for 'gtbl', 'EQ' and 'EN' for
'geqn', and 'PS' and 'PE' for 'gpic'.

3.2.14 Configuration and Customization
--------------------------------------

Packages provide means of customizing many of the details of how the
package behaves.  These range from setting the default type size to
changing the appearance of section headers.

4 Macro Packages
****************

This chapter surveys the "major" macro packages that come with 'groff'.
One, 'ms', is presented in detail.

   Major macro packages are also sometimes described as "full-service"
due to the breadth of features they provide and because more than one
cannot be used by the same document; for example

     groff -m man foo.man -m ms bar.doc

doesn't work.  Option arguments are processed before non-option
arguments; the above (failing) sample is thus reordered to

     groff -m man -m ms foo.man bar.doc

   Many auxiliary, or "minor", macro packages are also available.  They
may in general be used with any full-service macro package and handle a
variety of tasks from character encoding selection, to language
localization, to inlining of raster images.  See the 'groff_tmac(5)' man
page for a list.  Type 'man groff_tmac' at the command line to view it.

4.1 'man'
=========

The 'man' macro package is the most widely used and probably the most
important ever developed for 'troff'.  It is easy to use, and a vast
majority of manual pages ("man pages") are written in it.

   'groff''s implementation is documented in the 'groff_man(7)' man
page.  Type 'man groff_man' at the command line to view it.

4.1.1 Optional 'man' extensions
-------------------------------

Use the file 'man.local' for local extensions to the 'man' macros or for
style changes.

Custom headers and footers
..........................

In 'groff' versions 1.18.2 and later, you can specify custom headers and
footers by redefining the following macros in 'man.local'.

 -- Macro: .PT
     Control the content of the headers.  Normally, the header prints
     the command name and section number on either side, and the
     optional fifth argument to 'TH' in the center.

 -- Macro: .BT
     Control the content of the footers.  Normally, the footer prints
     the page number and the third and fourth arguments to 'TH'.

     Use the 'FT' register to specify the footer position.  The default
     is -0.5i.

Ultrix-specific man macros
..........................

The 'groff' source distribution includes a file named 'man.ultrix',
containing macros compatible with the Ultrix variant of 'man'.  Copy
this file into 'man.local' (or use the 'mso' request to load it) to
enable the following macros.

 -- Macro: .CT key
     Print '<CTRL/KEY>'.

 -- Macro: .CW
     Print subsequent text using a "constant-width" (monospaced)
     typeface (Courier roman).

 -- Macro: .Ds
     Begin a non-filled display.

 -- Macro: .De
     End a non-filled display started with 'Ds'.

 -- Macro: .EX [indent]
     Begin a non-filled display using a monospaced typeface (Courier
     roman).  Use the optional INDENT argument to indent the display.

 -- Macro: .EE
     End a non-filled display started with 'EX'.

 -- Macro: .G [text]
     Set TEXT in Helvetica.  If no text is present on the line where the
     macro is called, then the text of the next line appears in
     Helvetica.

 -- Macro: .GL [text]
     Set TEXT in Helvetica oblique.  If no text is present on the line
     where the macro is called, then the text of the next line appears
     in Helvetica Oblique.

 -- Macro: .HB [text]
     Set TEXT in Helvetica bold.  If no text is present on the line
     where the macro is called, then all text up to the next 'HB'
     appears in Helvetica bold.

 -- Macro: .TB [text]
     Identical to 'HB'.

 -- Macro: .MS title sect [punct]
     Set a man page reference in Ultrix format.  The TITLE is in Courier
     instead of italic.  Optional punctuation follows the section number
     without an intervening space.

 -- Macro: .NT [C] [title]
     Begin a note.  Print the optional title, or the word "Note",
     centered on the page.  Text following the macro makes up the body
     of the note, and is indented on both sides.  If the first argument
     is 'C', the body of the note is printed centered (the second
     argument replaces the word "Note" if specified).

 -- Macro: .NE
     End a note begun with 'NT'.

 -- Macro: .PN path [punct]
     Set the path name in a monospaced typeface (Courier roman),
     followed by optional punctuation.

 -- Macro: .Pn [punct] path [punct]
     If called with two arguments, identical to 'PN'.  If called with
     three arguments, set the second argument in a monospaced typeface
     (Courier roman), bracketed by the first and third arguments in the
     current font.

 -- Macro: .R
     Switch to roman font and turn off any underlining in effect.

 -- Macro: .RN
     Print the string '<RETURN>'.

 -- Macro: .VS [4]
     Start printing a change bar in the margin if the number '4' is
     specified.  Otherwise, this macro does nothing.

 -- Macro: .VE
     End printing the change bar begun by 'VS'.

Simple example
..............

The following example 'man.local' file alters the 'SH' macro to add some
extra vertical space before printing the heading.  Headings are printed
in Helvetica bold.

     .\" Make the heading fonts Helvetica
     .ds HF HB
     .
     .\" Put more space in front of headings.
     .rn SH SH-orig
     .de SH
     .  if t .sp (u;\\n[PD]*2)
     .  SH-orig \\$*
     ..

4.2 'mdoc'
==========

'groff''s implementation of the BSD 'doc' package for man pages is
documented in the 'groff_mdoc(7)' man page.  Type 'man groff_mdoc' at
the command line to view it.

4.3 'me'
========

'groff''s implementation of the BSD 'me' macro package is documented
using itself.  A tutorial, 'meintro.me', and reference, 'meref.me', are
available in 'groff''s documentation directory.  A 'groff_me(7)' man
page is also available and identifies the installation path for these
documents.  Type 'man groff_me' at the command line to view it.

   A French translation of the tutorial is available as 'meintro_fr.me'
and installed parallel to the English version.

4.4 'mm'
========

'groff''s implementation of the AT&T memorandum macro package is
documented in the 'groff_mm(7)' man page.  Type 'man groff_mm' at the
command line) to view it.

   A Swedish localization of 'mm' is also available; see
'groff_mmse(7)'.

4.5 'mom'
=========

The main documentation files for the 'mom' macros are in HTML format.
Additional, useful documentation is in PDF format.  See the 'groff(1)'
man page, section "Installation Directories", for their location.

   * 'toc.html' Entry point to the full mom manual.

   * 'macrolist.html' Hyperlinked index of macros with brief
     descriptions, arranged by category.

   * 'mom-pdf.pdf' PDF features and usage.

   The mom macros are in active development between 'groff' releases.
The most recent version, along with up-to-date documentation, is
available at <http://www.schaffter.ca/mom/mom-05.html>.

   The 'groff_mom(7)' man page (type 'man groff_mom' at the command
line) contains a partial list of available macros, however their usage
is best understood by consulting the HTML documentation.

4.6 'ms'
========

The 'ms' ("manuscript") package is suitable for the preparation of
letters, memoranda, reports, and books.  These 'groff' macros feature
cover page and table of contents generation, automatically numbered
headings, several paragraph styles, a variety of text styling options,
footnotes, and multi-column page layouts.  'ms' supports the 'tbl',
'eqn', 'pic', and 'refer' preprocessors for inclusion of tables,
mathematical equations, diagrams, and standardized bibliographic
citations.  This implementation is mostly compatible with the documented
interface and behavior of AT&T Unix Version 7 'ms'.  Many extensions
from 4.2BSD (Berkeley) and Tenth Edition Research Unix have been
recreated.

4.6.1 Introduction
------------------

The 'ms' macros are the oldest surviving package for 'roff' systems.(1)
(*note ms Introduction-Footnote-1::) While the 'man' package was
designed for brief reference documents, the 'ms' macros are also
suitable for longer works intended for printing and possible
publication.

   (1) While manual _pages_ are older, early ones used macros supplanted
by the 'man' package of Seventh Edition Unix (1979).  'ms' shipped with
Sixth Edition (1975) and was documented by Mike Lesk in a Bell Labs
internal memorandum.

4.6.1.1 Basic information
.........................

'ms' documents are plain text files; prepare them with your preferred
text editor.  If you're in a hurry to start, know that 'ms' needs one of
its macros called at the beginning of a document so that it can
initialize.  A "macro" is a formatting instruction to 'ms'.  Put a macro
call on a line by itself.  Use '.PP' if you want your paragraph's first
line to be indented, or '.LP' if you don't.

   After that, start typing normally.  It is a good practice to start
each sentence on a new line, or to put two spaces after sentence-ending
punctuation, so that the formatter knows where the sentence boundaries
are.  You can separate paragraphs with further paragraphing macros, or
with blank lines, and you can indent with tabs.  When you need one of
the features mentioned earlier (*note ms::), return to this part of the
manual.

   Format the document with the 'groff' command.  'nroff' can be useful
for previewing.

     $ editor radical.ms
     $ nroff -ww -z -ms radical.ms # check for errors
     $ nroff -ms radical.ms | less -R
     $ groff -T ps -ms radical.ms > radical.ps
     $ see radical.ps

   Our 'radical.ms' document might look like this.

     .LP
     Radical novelties are so disturbing that they tend to be
     suppressed or ignored, to the extent that even the
     possibility of their existence in general is more often
     denied than admitted.

     ->That's what Dijkstra said, anyway.

   'ms' exposes many aspects of document layout to user control via
'groff''s "registers" and "strings", which store numbers and text,
respectively.  Measurements in 'groff' are expressed with a suffix
called a "scaling unit".

'i'
     inches

'c'
     centimeters

'p'
     points (1/72 inch)

'P'
     picas (1/6 inch)

'v'
     vees; current vertical spacing

'm'
     ems; width of an "M" in the current font

'n'
     ens; one-half em

   Set registers with the 'nr' request and strings with the 'ds'
request.  "Requests" are like macro calls; they go on lines by
themselves and start with the "control character", a dot ('.').  The
difference is that they directly instruct the formatter program, rather
than the macro package.  We'll discuss a few as applicable.  It is wise
to specify a scaling unit when setting any register that represents a
length, size, or distance.

     .nr PS 10.5p \" Use 10.5-point type.
     .ds FAM P    \" Use Palatino font family.

In the foregoing, we see that '\"' begins a comment.  This is an example
of an "escape sequence", the other kind of formatting instruction.
Escape sequences can appear anywhere.  They begin with the escape
character ('\') and are followed by at least one more character.  'ms'
documents tend to use only a few of 'groff''s many requests and escape
sequences; see *note Request Index:: and *note Escape Sequence Index::
or the 'groff(7)' man page for complete lists.

'\"'
     Begin comment; ignore remainder of line.

'\n[REG]'
     Interpolate value of register REG.

'\*[STR]'
     Interpolate contents of string STR.

'\*S'
     abbreviation of '\*[S]'; the name S must be only one character

'\[CHAR]'
     Interpolate glyph of special character named CHAR.

'\&'
     dummy character

'\~'
     Insert an unbreakable space that is adjustable like a normal space.

'\|'
     Move horizontally by one-sixth em ("thin space").

   Prefix any words that start with a dot '.' or neutral apostrophe '''
with '\&' if they are at the beginning of an input line (or might become
that way in editing) to prevent them from being interpreted as macro
calls or requests.  Suffix '.', '?', and '!' with '\&' when needed to
cancel end-of-sentence detection.

     My exposure was \&.5 to \&.6 Sv of neutrons, said Dr.\&
     Wallace after the criticality incident.

4.6.2 Document Structure
------------------------

The 'ms' macro package expects a certain amount of structure: a
well-formed document contains at least one paragraphing or heading macro
call.  Longer documents have a structure as follows.

*Document type*
     Calling the 'RP' macro at the beginning of your document puts the
     document description (see below) on a cover page.  Otherwise, 'ms'
     places the information (if any) on the first page, followed
     immediately by the body text.  Some document types found in other
     'ms' implementations are specific to AT&T or Berkeley, and are not
     supported by 'groff' 'ms'.

*Format and layout*
     By setting registers and strings, you can configure your document's
     typeface, margins, spacing, headers and footers, and footnote
     arrangement.  *Note ms Document Control Settings::.

*Document description*
     A document description consists of any of: a title, one or more
     authors' names and affiliated institutions, an abstract, and a date
     or other identifier.  *Note ms Document Description Macros::.

*Body text*
     The main matter of your document follows its description (if any).
     'ms' supports highly structured text consisting of paragraphs
     interspersed with multi-level headings (chapters, sections,
     subsections, and so forth) and augmented by lists, footnotes,
     tables, diagrams, and similar material.  *Note ms Body Text::.

*Tables of contents*
     Macros enable the collection of entries for a table of contents (or
     index) as the material they discuss appears in the document.  You
     then call a macro to emit the table of contents at the end of your
     document.  The table of contents must necessarily follow the rest
     of the text since GNU 'troff' is a single-pass formatter; it thus
     cannot determine the page number of a division of the text until it
     has been set and output.  Since 'ms' was designed for the
     production of hard copy, the traditional procedure was to manually
     relocate the pages containing the table of contents between the
     cover page and the body text.  Today, page resequencing is more
     often done in the digital domain.  An index works similarly, but
     because it typically needs to be sorted after collection, its
     preparation requires separate processing.

4.6.3 Document Control Settings
-------------------------------

'ms' exposes many aspects of document layout to user control via 'groff'
requests.  To use them, you must understand how to define registers and
strings.

 -- Request: .nr reg value
     Set register REG to VALUE.  If REG doesn't exist, GNU 'troff'
     creates it.

 -- Request: .ds name contents
     Set string NAME to CONTENTS.

   A list of document control registers and strings follows.  For any
parameter whose default is unsatisfactory, define its register or string
before calling any 'ms' macro other than 'RP'.

Margin settings
...............

 -- Register: \n[PO]
     Defines the page offset (i.e., the left margin).

     Effective: next page.

     Default: Varies by output device and paper format; 1i is used for
     typesetters using U.S. letter paper, and zero for terminals.  *Note
     Paper Format::.

 -- Register: \n[LL]
     Defines the line length (i.e., the width of the body text).

     Effective: next paragraph.

     Default: Varies by output device and paper format; 6.5i is used for
     typesetters using U.S. letter paper (*note Paper Format::) and 65n
     on terminals.

 -- Register: \n[LT]
     Defines the title line length (i.e., the header and footer width).
     This is usually the same as 'LL', but need not be.

     Effective: next paragraph.

     Default: Varies by output device and paper format; 6.5i is used for
     typesetters using U.S. letter paper (*note Paper Format::) and 65n
     on terminals.

 -- Register: \n[HM]
     Defines the header margin height at the top of the page.

     Effective: next page.

     Default: 1i.

 -- Register: \n[FM]
     Defines the footer margin height at the bottom of the page.

     Effective: next page.

     Default: 1i.

Titles (headers, footers)
.........................

 -- String: \*[LH]
     Defines the text displayed in the left header position.

     Effective: next header.

     Default: empty.

 -- String: \*[CH]
     Defines the text displayed in the center header position.

     Effective: next header.

     Default: '-\n[%]-'.

 -- String: \*[RH]
     Defines the text displayed in the right header position.

     Effective: next header.

     Default: empty.

 -- String: \*[LF]
     Defines the text displayed in the left footer position.

     Effective: next footer.

     Default: empty.

 -- String: \*[CF]
     Defines the text displayed in the center footer position.

     Effective: next footer.

     Default: empty.

 -- String: \*[RF]
     Defines the text displayed in the right footer position.

     Effective: next footer.

     Default: empty.

Text settings
.............

 -- Register: \n[PS]
     Defines the type size of the body text.

     Effective: next paragraph.

     Default: 10p.

 -- Register: \n[VS]
     Defines the vertical spacing (type size plus leading).

     Effective: next paragraph.

     Default: 12p.

 -- Register: \n[HY]
     Defines the automatic hyphenation mode used with the 'hy' request.
     Setting 'HY' to 0 is equivalent to using the 'nh' request.  This is
     a Tenth Edition Research Unix extension.

     Effective: next paragraph.

     Default: 6.

 -- String: \*[FAM]
     Defines the font family used to typeset the document.  This is a
     GNU extension.

     Effective: next paragraph.

     Default: defined by the output device; often 'T' (*note ms Body
     Text::)

Paragraph settings
..................

 -- Register: \n[PI]
     Defines the indentation amount used by the 'PP', 'IP' (unless
     overridden by an optional argument), 'XP', and 'RS' macros.

     Effective: next paragraph.

     Default: 5n.

 -- Register: \n[PD]
     Defines the space between paragraphs.

     Effective: next paragraph.

     Default: 0.3v (1v on low-resolution devices).

 -- Register: \n[QI]
     Defines the indentation amount used on both sides of a paragraph
     set with the 'QP' or between the 'QS' and 'QE' macros.

     Effective: next paragraph.

     Default: 5n.

 -- Register: \n[PORPHANS]
     Defines the minimum number of initial lines of any paragraph that
     must be kept together to avoid isolated lines at the bottom of a
     page.  If a new paragraph is started close to the bottom of a page,
     and there is insufficient space to accommodate 'PORPHANS' lines
     before an automatic page break, then a page break is forced before
     the start of the paragraph.  This is a GNU extension.

     Effective: next paragraph.

     Default: 1.

Heading settings
................

 -- Register: \n[PSINCR]
     Defines an increment in type size to be applied to a heading at a
     lesser depth than that specified in 'GROWPS'.  The value of
     'PSINCR' should be specified in points with the p scaling unit and
     may include a fractional component; for example, '.nr PSINCR 1.5p'
     sets a type size increment of 1.5p.  This is a GNU extension.

     Effective: next heading.

     Default: 1p.

 -- Register: \n[GROWPS]
     Defines the heading depth above which the type size increment set
     by 'PSINCR' becomes effective.  For each heading depth less than
     the value of 'GROWPS', the type size is increased by 'PSINCR'.
     Setting 'GROWPS' to any value less than 2 disables the incremental
     heading size feature.  This is a GNU extension.

     Effective: next heading.

     Default: 0.

 -- Register: \n[HORPHANS]
     Defines the minimum number of lines of an immediately succeeding
     paragraph that should be kept together with any heading introduced
     by the 'NH' or 'SH' macros.  If a heading is placed close to the
     bottom of a page, and there is insufficient space to accommodate
     both the heading and at least 'HORPHANS' lines of the following
     paragraph, before an automatic page break, then the page break is
     forced before the heading.  This is a GNU extension.

     Effective: next paragraph.

     Default: 1.

 -- String: \*[SN-STYLE]
     Defines the style used to print numbered headings.  *Note Headings
     in ms::.  This is a GNU extension.

     Effective: next heading.

     Default: alias of 'SN-DOT'

Footnote settings
.................

 -- Register: \n[FI]
     Defines the footnote indentation.  This is a Berkeley extension.

     Effective: next footnote.

     Default: 2n.

 -- Register: \n[FF]
     Defines the format of automatically numbered footnotes, and those
     for which the 'FS' request is given a marker argument, at the
     bottom of a column or page.  This is a Berkeley extension.
     '0'
          Set an automatic number(1) (*note ms Document Control
          Settings-Footnote-1::) as a superscript (on typesetter
          devices) or surrounded by square brackets (on terminals).  The
          footnote paragraph is indented as with 'PP' if there is an
          'FS' argument or an automatic number, and as with 'LP'
          otherwise.  This is the default.

     '1'
          As '0', but set the marker as regular text and follow an
          automatic number with a period.

     '2'
          As '1', but without indentation (like 'LP').

     '3'
          As '1', but set the footnote paragraph with the marker hanging
          (like 'IP').

     Effective: next footnote.

     Default: 0.

 -- Register: \n[FPS]
     Defines the footnote type size.

     Effective: next footnote.

     Default: '\n[PS] - 2p'.

 -- Register: \n[FVS]
     Defines the footnote vertical spacing.

     Effective: next footnote.

     Default: '\n[FPS] + 2p'.

 -- Register: \n[FPD]
     Defines the footnote paragraph spacing.  This is a GNU extension.

     Effective: next footnote.

     Default: '\n[PD] / 2'.

 -- String: \*[FR]
     Defines the ratio of the footnote line length to the current line
     length.  This is a GNU extension.

     Effective: next footnote in single-column arrangements, next page
     otherwise.

     Default: '11/12'.

Display settings
................

 -- Register: \n[DD]
     Sets the display distance--the vertical spacing before and after a
     display, a 'tbl' table, an 'eqn' equation, or a 'pic' image.  This
     is a Berkeley extension.

     Effective: next display boundary.

     Default: 0.5v (1v on low-resolution devices).

 -- Register: \n[DI]
     Sets the default amount by which to indent a display started with
     'DS' and 'ID' without arguments, to '.DS I' without an indentation
     argument, and to equations set with '.EQ I'.  This is a GNU
     extension.

     Effective: next indented display.

     Default: 0.5i.

Other settings
..............

 -- Register: \n[MINGW]
     Defines the default minimum width between columns in a multi-column
     document.  This is a GNU extension.

     Effective: next page.

     Default: 2n.

 -- Register: \n[TC-MARGIN]
     Defines the width of the field in which page numbers are set in a
     table of contents entry; the right margin thus moves inboard by
     this amount.  This is a GNU extension.

     Effective: next 'PX' call.

     Default: '\w'000''

   (1) defined in *note ms Footnotes::

4.6.4 Document Description Macros
---------------------------------

Only the simplest document lacks a title.(1)  (*note ms Document
Description Macros-Footnote-1::) As its level of sophistication (or
complexity) increases, it tends to acquire a date of revision,
explicitly identified authors, sponsoring institutions for authors, and,
at the rarefied heights, an abstract of its content.  Define these data
by calling the macros below in the order shown; 'DA' or 'ND' can be
called to set the document date (or other identifier) at any time before
(a) the abstract, if present, or (b) its information is required in a
header or footer.  Use of these macros is optional, except that 'TL' is
mandatory if any of 'RP', 'AU', 'AI', or 'AB' is called, and 'AE' is
mandatory if 'AB' is called.

 -- Macro: .RP [no-repeat-info] [no-renumber]
     Use the "report" (AT&T: "released paper") format for your document,
     creating a separate cover page.  The default arrangement is to
     place most of the document description (title, author names and
     institutions, and abstract, but not the date) at the top of the
     first page.  If the optional 'no-repeat-info' argument is given,
     'ms' produces a cover page but does not repeat any of its
     information subsequently (but see the 'DA' macro below regarding
     the date).  Normally, 'RP' sets the page number following the cover
     page to 1.  Specifying the optional 'no-renumber' argument
     suppresses this alteration.  Optional arguments can occur in any
     order.  'no' is recognized as a synonym of 'no-repeat-info' for
     'AT&T' compatibility.

 -- Macro: .TL
     Specify the document title.  'ms' collects text on input lines
     following this call into the title until reaching 'AU', 'AB', or a
     heading or paragraphing macro call.

 -- Macro: .AU
     Specify an author's name.  'ms' collects text on input lines
     following this call into the author's name until reaching 'AI',
     'AB', another 'AU', or a heading or paragraphing macro call.  Call
     it repeatedly to specify multiple authors.

 -- Macro: .AI
     Specify the preceding author's institution.  An 'AU' call is
     usefully followed by at most one 'AI' call; if there are more, the
     last 'AI' call controls.  'ms' collects text on input lines
     following this call into the author's institution until reaching
     'AU', 'AB', or a heading or paragraphing macro call.

 -- Macro: .DA [x ...]
     Typeset the current date, or any arguments X, in the center footer,
     and, if 'RP' is also called, left-aligned at the end of the
     description information on the cover page.

 -- Macro: .ND [x ...]
     Typeset the current date, or any arguments X, if 'RP' is also
     called, left-aligned at the end of the document description on the
     cover page.  This is 'groff' 'ms''s default.

 -- Macro: .AB [no]
     Begin the abstract.  'ms' collects text on input lines following
     this call into the abstract until reaching an 'AE' call.  By
     default, 'ms' places the word "ABSTRACT" centered and in italics
     above the text of the abstract.  The optional argument 'no'
     suppresses this heading.

 -- Macro: .AE
     End the abstract.

   An example document description, using a cover page, follows.

     .RP
     .TL
     The Inevitability of Code Bloat
     in Commercial and Free Software
     .AU
     J.\& Random Luser
     .AI
     University of West Bumblefuzz
     .AB
     This report examines the long-term growth of the code
     bases in two large,
     popular software packages;
     the free Emacs and the commercial Microsoft Word.
     While differences appear in the type or order of
     features added,
     due to the different methodologies used,
     the results are the same in the end.
     .PP
     The free software approach is shown to be superior in
     that while free software can become as bloated as
     commercial offerings,
     free software tends to have fewer serious bugs and the
     added features are more in line with user demand.
     .AE

     ...the rest of the paper...

   (1) Distinguish a document title from "titles", which are what 'roff'
systems call headers and footers collectively.

4.6.5 Body Text
---------------

A variety of macros, registers, and strings can be used to structure and
style the body of your document.  They organize your text into
paragraphs, headings, footnotes, and inclusions of material such as
tables and figures.

4.6.5.1 Text settings
.....................

The 'FAM' string, a GNU extension, sets the font family for body text;
the default is 'T'.  The 'PS' and 'VS' registers set the type size and
vertical spacing (distance between text baselines), respectively.  The
font family and type size are ignored on terminal devices.  Setting
these parameters before the first call of a heading, paragraphing, or
(non-date) document description macro also applies them to headers,
footers, and (for 'FAM') footnotes.

   Which font families are available depends on the output device; as a
convention, 'T' selects a serif family ("Times"), 'H' a sans-serif
family ("Helvetica"), and 'C' a monospaced family ("Courier").  The man
page for the output driver documents its font repertoire.  Consult the
'groff(1)' man page for lists of available output devices and their
drivers.

   The hyphenation mode (as used by the 'hy' request) is set from the
'HY' register.  Setting 'HY' to '0' is equivalent to using the 'nh'
request.  This is a Tenth Edition Research Unix extension.

4.6.5.2 Typographical symbols
.............................

'ms' provides a few strings to obtain typographical symbols not easily
entered with the keyboard.  These and many others are available as
special character escape sequences--see the 'groff_char(7)' man page.

 -- String: \*[-]
     Interpolate an em dash.

 -- String: \*[Q]
 -- String: \*[U]
     Interpolate typographer's quotation marks where available, and
     neutral double quotes otherwise.  '\*Q' is the left quote and '\*U'
     the right.

4.6.5.3 Paragraphs
..................

Paragraphing macros "break", or terminate, any pending output line so
that a new paragraph can begin.  Several paragraph types are available,
differing in how indentation applies to them: to left, right, or both
margins; to the first output line of the paragraph, all output lines, or
all but the first.  All paragraphing macro calls cause the insertion of
vertical space in the amount stored in the 'PD' register, except at page
or column breaks.  Alternatively, a blank input line breaks the output
line and vertically spaces by one vee.

 -- Macro: .LP
     Set a paragraph without any (additional) indentation.

 -- Macro: .PP
     Set a paragraph with a first-line left indentation in the amount
     stored in the 'PI' register.

 -- Macro: .IP [marker [width]]
     Set a paragraph with a left indentation.  The optional MARKER is
     not indented and is empty by default.  It has several applications;
     see *note Lists in ms::.  WIDTH overrides the indentation amount
     stored in the 'PI' register; its default unit is 'n'.  Once
     specified, WIDTH applies to further 'IP' calls until specified
     again or a heading or different paragraphing macro is called.

 -- Macro: .QP
     Set a paragraph indented from both left and right margins by the
     amount stored in the 'QI' register.

 -- Macro: .QS
 -- Macro: .QE
     Begin ('QS') and end ('QE') a region where each paragraph is
     indented from both margins by the amount stored in the 'QI'
     register.  The text between 'QS' and 'QE' can be structured further
     by use of other paragraphing macros.

 -- Macro: .XP
     Set an "exdented" paragraph--one with a left indentation in the
     amount stored in the 'PI' register on every line _except_ the first
     (also known as a hanging indent).  This is a Berkeley extension.

   The following example illustrates the use of paragraphing macros.

     .NH 2
     Cases used in the 2001 study
     .LP
     Two software releases were considered for this report.
     .PP
     The first is commercial software;
     the second is free.
     .IP \[bu]
     Microsoft Word for Windows,
     starting with version 1.0 through the current version
     (Word 2000).
     .IP \[bu]
     GNU Emacs,
     from its first appearance as a standalone editor through
     the current version (v20).
     See [Bloggs 2002] for details.
     .QP
     Franklin's Law applied to software:
     software expands to outgrow both RAM and disk space over
     time.
     .SH
     Bibliography
     .XP
     Bloggs, Joseph R.,
     .I "Everyone's a Critic" ,
     Underground Press, March 2002.
     A definitive work that answers all questions and
     criticisms about the quality and usability of free
     software.

4.6.5.4 Headings
................

Use headings to create a sequential or hierarchical structure for your
document.  The 'ms' macros print headings in *bold* using the same font
family and, by default, type size as the body text.  Headings are
available with and without automatic numbering.  Text on input lines
following the macro call becomes the heading's title.  Call a
paragraphing macro to end the heading text and start the section's
content.

 -- Macro: .NH [depth]
 -- Macro: .NH S heading-depth-index ...
     Set an automatically numbered heading.

     'ms' produces a numbered heading the form A.B.C..., to any depth
     desired, with the numbering of each depth increasing automatically
     and being reset to zero when a more significant level is increased.
     "1" is the most significant or coarsest division of the document.
     Only non-zero values are output.  If DEPTH is omitted, it is taken
     to be '1'.

     If you specify DEPTH such that an ascending gap occurs relative to
     the previous 'NH' call--that is, you "skip a depth", as by '.NH 1'
     and then '.NH 3'--'groff' 'ms' emits a warning on the standard
     error stream.

     Alternatively, you can give 'NH' a first argument of 'S', followed
     by integers to number the heading depths explicitly.  Further
     automatic numbering, if used, resumes using the specified indices
     as their predecessors.  This feature is a Berkeley extension.

   An example may be illustrative.

     .NH 1
     Animalia
     .NH 2
     Arthropoda
     .NH 3
     Crustacea
     .NH 2
     Chordata
     .NH S 6 6 6
     Daimonia
     .NH 1
     Plantae

   The above results in numbering as follows; the vertical space that
normally precedes each heading is omitted.

     1.  Animalia
     1.1.  Arthropoda
     1.1.1.  Crustacea
     1.2.  Chordata
     6.6.6.  Daimonia
     7.  Plantae

 -- String: \*[SN-STYLE]
 -- String: \*[SN-DOT]
 -- String: \*[SN-NO-DOT]
 -- String: \*[SN]
     After 'NH' is called, the assigned number is made available in the
     strings 'SN-DOT' (as it appears in a printed heading with default
     formatting, followed by a terminating period) and 'SN-NO-DOT' (with
     the terminating period omitted).  These are GNU extensions.

     You can control the style used to print numbered headings by
     defining an appropriate alias for the string 'SN-STYLE'.  By
     default, 'SN-STYLE' is aliased to 'SN-DOT'.  If you prefer to omit
     the terminating period from numbers appearing in numbered headings,
     you may define the alias as follows.

          .als SN-STYLE SN-NO-DOT

     Any such change in numbering style becomes effective from the next
     use of 'NH' following redefinition of the alias for 'SN-STYLE'.
     The formatted number of the current heading is available in the
     'SN' string (a feature first documented by Berkeley), which
     facilitates its inclusion in, for example, table captions, equation
     labels, and 'XS'/'XA'/'XE' table of contents entries.

 -- Macro: .SH [depth]
     Set an unnumbered heading.

     The optional DEPTH argument is a GNU extension indicating the
     heading depth corresponding to the DEPTH argument of 'NH'.  It
     matches the type size at which the heading is set to that of a
     numbered heading at the same depth when the 'GROWPS' and 'PSINCR'
     heading size adjustment mechanism is in effect.

   If the 'GROWPS' register is set to a value greater than the LEVEL
argument to 'NH' or 'SH', the type size of a heading produced by these
macros increases by 'PSINCR' units over the size specified by 'PS'
multiplied by the difference of 'GROWPS' and LEVEL.  The value stored in
'PSINCR' is interpreted in 'groff' basic units; the 'p' scaling unit
should be employed when assigning a value specified in points.  For
example, the sequence

     .nr PS 10
     .nr GROWPS 3
     .nr PSINCR 1.5p
     .NH 1
     Carnivora
     .NH 2
     Felinae
     .NH 3
     Felis catus
     .SH 2
     Machairodontinae

will cause "1.  Carnivora" to be printed in 13-point text, followed by
"1.1.  Felinae" in 11.5-point text, while "1.1.1.  Felis catus" and all
more deeply nested heading levels will remain in the 10-point text
specified by the 'PS' register.  "Machairodontinae" is printed at 11.5
points, since it corresponds to heading level 2.

   The 'HORPHANS' register operates in conjunction with the 'NH' and
'SH' macros to inhibit the printing of isolated headings at the bottom
of a page; it specifies the minimum number of lines of an immediately
subsequent paragraph that must be kept on the same page as the heading.
If insufficient space remains on the current page to accommodate the
heading and this number of lines of paragraph text, a page break is
forced before the heading is printed.  Any display macro call or 'tbl',
'pic', or 'eqn' region between the heading and the subsequent paragraph
suppresses this grouping.  *Note ms keeps and displays:: and *note ms
Insertions::.

4.6.5.5 Typeface and decoration
...............................

The 'ms' macros provide a variety of ways to style text.  Attend closely
to the ordering of arguments labeled PRE and POST, which is not
intuitive.  Support for PRE arguments is a GNU extension.(1)  (*note
Typeface and decoration-Footnote-1::)

 -- Macro: .B [text [post [pre]]]
     Style TEXT in bold, followed by POST in the previous font style
     without intervening space, and preceded by PRE similarly.  Without
     arguments, 'ms' styles subsequent text in bold until the next
     paragraphing, heading, or no-argument typeface macro call.

 -- Macro: .R [text [post [pre]]]
     As 'B', but use the roman style (upright text of normal weight)
     instead of bold.  Argument recognition is a GNU extension.

 -- Macro: .I [text [post [pre]]]
     As 'B', but use an italic or oblique style instead of bold.

 -- Macro: .BI [text [post [pre]]]
     As 'B', but use a bold italic or bold oblique style instead of
     upright bold.  This is a Tenth Edition Research Unix extension.

 -- Macro: .CW [text [post [pre]]]
     As 'B', but use a constant-width (monospaced) roman typeface
     instead of bold.  This is a Tenth Edition Research Unix extension.

 -- Macro: .BX [text]
     Typeset TEXT and draw a box around it.  On terminal devices,
     reverse video is used instead.  If you want TEXT to contain space,
     use unbreakable space or horizontal motion escape sequences ('\~',
     '\<SP>', '\^', '\|', '\0' or '\h').

 -- Macro: .UL [text [post]]
     Typeset TEXT with an underline.  POST, if present, is set after
     TEXT with no intervening space.

 -- Macro: .LG
     Set subsequent text in larger type (two points larger than the
     current size) until the next type size, paragraphing, or heading
     macro call.  You can specify this macro multiple times to enlarge
     the type size as needed.

 -- Macro: .SM
     Set subsequent text in smaller type (two points smaller than the
     current size) until the next type size, paragraphing, or heading
     macro call.  You can specify this macro multiple times to reduce
     the type size as needed.

 -- Macro: .NL
     Set subsequent text at the normal type size (the amount in the 'PS'
     register).

   PRE and POST arguments are typically used to simplify the attachment
of punctuation to styled words.  When PRE is used, a hyphenation control
escape sequence '\%' that would ordinarily start TEXT must start PRE
instead to have the desired effect.

     The CS course's students found one C language keyword
     .CW static ) \%(
     most troublesome.

   The foregoing example produces output as follows.

     The CS course's students found one C language keyword (static)
     most troublesome.

   You can use the output line continuation escape sequence '\c' to
achieve the same result (*note Line Continuation::).  It is also
portable to older 'ms' implementations.

     The CS course's students found one C language keyword
     \%(\c
     .CW \%static )
     most troublesome.

   'groff' 'ms' also offers strings to begin and end super- and
subscripting.  These are GNU extensions.

 -- String: \*[{]
 -- String: \*[}]
     Begin and end superscripting, respectively.

 -- String: \*[<]
 -- String: \*[>]
     Begin and end subscripting, respectively.

   Rather than calling the 'CW' macro, in 'groff' 'ms' you might prefer
to change the font family to Courier by setting the 'FAM' string to 'C'.
You can then use all four style macros above, returning to the default
family (Times) with '.ds FAM T'.  Because changes to 'FAM' take effect
only at the next paragraph, 'CW' remains useful to "inline" a change to
the font family, similarly to the practice of this document in noting
syntactical elements of 'ms' and 'groff'.

   (1) This idiosyncrasy arose through feature accretion; for example,
the 'B' macro in Version 6 Unix 'ms' (1975) accepted only one argument,
the text to be set in boldface.  By Version 7 (1979) it recognized a
second argument; in 1990, 'groff' 'ms' added a "pre" argument, placing
it third to avoid breaking support for older documents.

4.6.5.6 Lists
.............

The MARKER argument to the 'IP' macro can be employed to present a
variety of lists; for instance, you can use a bullet glyph ('\[bu]') for
unordered lists, a number (or auto-incrementing register) for numbered
lists, or a word or phrase for glossary-style or definition lists.  If
you set the paragraph indentation register 'PI' before calling 'IP', you
can later reorder the items in the list without having to ensure that a
WIDTH argument remains affixed to the first call.

   The following is an example of a bulleted list.

     .nr PI 2n
     A bulleted list:
     .IP \[bu]
     lawyers
     .IP \[bu]
     guns
     .IP \[bu]
     money

     A bulleted list:

     * lawyers

     * guns

     * money

   The following is an example of a numbered list.

     .nr step 0 1
     .nr PI 3n
     A numbered list:
     .IP \n+[step]
     lawyers
     .IP \n+[step]
     guns
     .IP \n+[step]
     money

     A numbered list:

     1. lawyers

     2. guns

     3. money

   Here we have employed the 'nr' request to create a register of our
own, 'step'.  We initialized it to zero and assigned it an
auto-increment of 1.  Each time we use the escape sequence '\n+[PI]'
(note the plus sign), the formatter applies the increment just before
interpolating the register's value.  Preparing the 'PI' register as well
enables us to rearrange the list without the tedium of updating macro
calls.

   The next example illustrates a glossary-style list.

     A glossary-style list:
     .IP lawyers 0.4i
     Two or more attorneys.
     .IP guns
     Firearms,
     preferably large-caliber.
     .IP money
     Gotta pay for those
     lawyers and guns!

     A glossary-style list:

     lawyers
           Two or more attorneys.

     guns  Firearms, preferably large-caliber.

     money
           Gotta pay for those lawyers and guns!

   In the previous example, observe how the 'IP' macro places the
definition on the same line as the term if it has enough space.  If this
is not what you want, there are a few workarounds we will illustrate by
modifying the example.  First, you can use a 'br' request to force a
break after printing the term or label.

     .IP guns
     .br
     Firearms,

   Second, you could apply the '\p' escape sequence to force a break.
The space following the escape sequence is important; if you omit it,
'groff' prints the first word of the paragraph text on the same line as
the term or label (if it fits) _then_ breaks the line.

     .IP guns
     \p Firearms,

   Finally, you may append a horizontal motion to the marker with the
'\h' escape sequence; using the same amount as the indentation will
ensure that the marker is too wide for 'groff' to treat it as "fitting"
on the same line as the paragraph text.

     .IP guns\h'0.4i'
     Firearms,

   In each case, the result is the same.

     A glossary-style list:

     lawyers
           Two or more attorneys.

     guns
           Firearms, preferably large-caliber.

     money
           Gotta pay for those lawyers and guns!

4.6.5.7 Indented regions
........................

You may need to indent a region of text while otherwise formatting it
normally.  Indented regions can be nested; you can change '\n[PI]'
before each call to vary the amount of inset.

 -- Macro: .RS
     Begin a region where headings, paragraphs, and displays are
     indented (further) by the amount stored in the 'PI' register.

 -- Macro: .RE
     End the (next) most recent indented region.

   This feature enables you to easily line up text under hanging and
indented paragraphs.  For example, you may wish to structure lists
hierarchically.

     .IP \[bu] 2
     Lawyers:
     .RS
     .IP \[bu]
     Dewey,
     .IP \[bu]
     Cheatham,
     and
     .IP \[bu]
     and Howe.
     .RE
     .IP \[bu]
     Guns

     * Lawyers:

       *  Dewey,

       *  Cheatham, and

       *  Howe.

     * Guns

4.6.5.8 Keeps, boxed keeps, and displays
........................................

On occasion, you may want to "keep" several lines of text, or a region
of a document, together on a single page, preventing an automatic page
break within certain boundaries.  This can cause a page break to occur
earlier than it normally would.  For example, you may want to keep two
paragraphs together, or a paragraph that refers to a table, list, or
figure adjacent to the item it discusses.  'ms' provides the 'KS' and
'KE' macros for this purpose.

   You can alternatively specify a "floating keep": if a keep cannot fit
on the current page, 'ms' holds its contents and allows material
following the keep (in the source document) to fill the remainder of the
current page.  When the page breaks, whether by reaching the end or 'bp'
request, 'ms' puts the floating keep at the beginning of the next page.
This is useful for placing large graphics or tables that do not need to
appear exactly where they occur in the source document.

 -- Macro: .KS
 -- Macro: .KF
 -- Macro: .KE
     'KS' begins a keep, 'KF' a floating keep, and 'KE' ends a keep of
     either kind.

   As an alternative to the keep mechanism, the 'ne' request forces a
page break if there is not at least the amount of vertical space
specified in its argument remaining on the page (*note Page Control::).
One application of 'ne' is to reserve space on the page for a figure or
illustration to be included later.

   A "boxed keep" has a frame drawn around it.

 -- Macro: .B1
 -- Macro: .B2
     'B1' begins a keep with a box drawn around it.  'B2' ends a boxed
     keep.

   Boxed keep macros cause breaks; if you need to box a word or phrase
within a line, see the 'BX' macro in *note Typeface and decoration::.
Box lines are drawn as close as possible to the text they enclose so
that they are usable within paragraphs.  If you wish to box one or more
paragraphs, you may improve the appearance by calling 'B1' after the
first paragraphing macro, and by adding a small amount of vertical space
before calling 'B2'.

     .LP
     .B1
     .I Warning:
     Happy Fun Ball may suddenly accelerate to dangerous
     speeds.
     .sp \n[PD]/2 \" space by half the inter-paragraph distance
     .B2

   If you want a boxed keep to float, you will need to enclose the 'B1'
and 'B2' calls within a pair of 'KF' and 'KE' calls.

   "Displays" turn off filling; lines of verse or program code are shown
with their lines broken as in the source document without requiring 'br'
requests between lines.  Displays can be kept on a single page or
allowed to break across pages.  The 'DS' macro begins a kept display of
the layout specified in its first argument; non-kept displays are begun
with dedicated macros corresponding to their layout.

 -- Macro: .DS L
 -- Macro: .LD
     Begin ('DS': kept) left-aligned display.

 -- Macro: .DS [I [indent]]
 -- Macro: .ID [indent]
     Begin ('DS': kept) display indented by INDENT if specified, and by
     the amount of the 'DI' register otherwise.

 -- Macro: .DS B
 -- Macro: .BD
     Begin a ('DS': kept) a block display: the entire display is
     left-aligned, but indented such that the longest line in the
     display is centered on the page.

 -- Macro: .DS C
 -- Macro: .CD
     Begin a ('DS': kept) centered display: each line in the display is
     centered.

 -- Macro: .DS R
 -- Macro: .RD
     Begin a ('DS': kept) right-aligned display.  This is a GNU
     extension.

 -- Macro: .DE
     End any display.

   The distance stored in the 'DD' register is inserted before and after
each pair of display macros; this is a Berkeley extension.  In 'groff'
'ms', this distance replaces any adjacent inter-paragraph distance or
subsequent spacing prior to a section heading.  The 'DI' register is a
GNU extension; its value is an indentation applied to displays created
with '.DS' and '.ID' without arguments, to '.DS I' without an
indentation argument, and to indented equations set with '.EQ'.  Changes
to either register take effect at the next display boundary.

4.6.5.9 Tables, figures, equations, and references
..................................................

The 'ms' package is often used with the 'tbl', 'pic', 'eqn', and 'refer'
preprocessors.  Mark text meant for preprocessors by enclosing it in
pairs of tokens as follows, with nothing between the dot and the macro
name.  The preprocessors match these tokens only at the start of an
input line.

 -- Macro: .TS [H]
 -- Macro: .TE
     Demarcate a table to be processed by the 'tbl' preprocessor.  The
     optional argument 'H' to 'TS' instructs 'ms' to repeat table rows
     (often column headings) at the top of each new page the table
     spans, if applicable; calling the 'TH' macro marks the end of such
     rows.  The GNU 'tbl(1)' man page provides a comprehensive reference
     to the preprocessor and offers examples of its use.

 -- Macro: .PS
 -- Macro: .PE
 -- Macro: .PF
     'PS' begins a picture to be processed by the 'gpic' preprocessor;
     either of 'PE' or 'PF' ends it, the latter with "flyback" to the
     vertical position at its top.  You can create 'pic' input manually
     or with a program such as 'xfig'.

 -- Macro: .EQ [align [label]]
 -- Macro: .EN
     Demarcate an equation to be processed by the 'eqn' preprocessor.
     The equation is centered by default; ALIGN can be 'C', 'L', or 'I'
     to (explicitly) center, left-align, or indent it by the amount
     stored in the 'DI' register, respectively.  If specified, LABEL is
     set right-aligned.

 -- Macro: .[
 -- Macro: .]
     Demarcate a bibliographic citation to be processed by the 'refer'
     preprocessor.  The GNU 'refer(1)' man page provides a comprehensive
     reference to the preprocessor and the format of its bibliographic
     database.  Type 'man refer' at the command line to view it.

   When 'refer' emits collected references (as might be done on a "Works
Cited" page), it interpolates the 'REFERENCES' string as an unnumbered
heading ('SH').

   The following is an example of how to set up a table that may print
across two or more pages.

     .TS H
     allbox;
     Cb | Cb .
     Part->Description
     _
     .TH
     .T&
     GH-1978->Fribulating gonkulator
     ...the rest of the table follows...
     .TE

Attempting to place a multi-page table inside a keep can lead to
unpleasant results, particularly if the 'tbl' 'allbox' option is used.

   Mathematics can be typeset using the language of the 'eqn'
preprocessor.

     .EQ C (\*[SN-NO-DOT]a)
     p ~ = ~ q sqrt { ( 1 + ~ ( x / q sup 2 ) }
     .EN

This input formats a labelled equation.  We used the 'SN-NO-DOT' string
to base the equation label on the current heading number, giving us more
flexibility to reorganize the document.

   Use 'groff' options to run preprocessors on the input: '-e' for
'geqn', '-p' for 'gpic', '-R' for 'grefer', and '-t' for 'gtbl'.

4.6.5.10 Footnotes
..................

A footnote is typically anchored to a place in the text with a "marker",
which is a small integer, a symbol such as a dagger, or arbitrary
user-specified text.

 -- String: \*[*]
     Place an "automatic number", an automatically generated numeric
     footnote marker, in the text.  Each time this string is
     interpolated, the number it produces increments by one.  Automatic
     numbers start at 1.  This is a Berkeley extension.

   Enclose the footnote text in 'FS' and 'FE' macro calls to set it at
the nearest available "foot", or bottom, of a text column or page.

 -- Macro: .FS [marker]
 -- Macro: .FE
     Begin ('FS') and end ('FE') a footnote.  'FS' calls 'FS-MARK' with
     any supplied MARKER argument, which is then also placed at the
     beginning of the footnote text.  If MARKER is omitted, the next
     pending automatic footnote number enqueued by interpolation of the
     '*' string is used, and if none exists, nothing is prefixed.

   You may not desire automatically numbered footnotes in spite of their
convenience.  You can indicate a footnote with a symbol or other text by
specifying its marker at the appropriate place (for example, by using
'\[dg]' for the dagger glyph) _and_ as an argument to the 'FS' macro.
Such manual marks should be repeated as arguments to 'FS' or as part of
the footnote text to disambiguate their correspondence.  You may wish to
use '\*{' and '\*}' to superscript the marker at the anchor point, in
the footnote text, or both.

   'groff' 'ms' provides a hook macro, 'FS-MARK', for user-determined
operations to be performed when the 'FS' macro is called.  It is passed
the same arguments as 'FS' itself.  An application of 'FS-MARK' is
anchor placement for a hyperlink reference, so that a footnote can link
back to its referential context.(1)  (*note ms Footnotes-Footnote-1::)
By default, this macro has an empty definition.  'FS-MARK' is a GNU
extension.

   Footnotes can be safely used within keeps and displays, but you
should avoid using automatically numbered footnotes within floating
keeps.  You can place a second '\**' interpolation between a '\**' and
its corresponding 'FS' call as long as each 'FS' call occurs _after_ the
corresponding '\**' and occurrences of 'FS' are in the same order as
corresponding occurrences of '\**'.

   Footnote text is formatted as paragraphs are, using analogous
parameters.  The registers 'FI', 'FPD', 'FPS', and 'FVS' correspond to
'PI', 'PD', 'PS', and 'CS', respectively; 'FPD', 'FPS', and 'FVS' are
GNU extensions.

   The 'FF' register controls the formatting of automatically numbered
footnote paragraphs and those for which 'FS' is given a marker argument.
*Note ms Document Control Settings::.

   The default footnote line length is 11/12ths of the normal line
length for compatibility with the expectations of historical 'ms'
documents; you may wish to set the 'FR' string to '1' to align with
contemporary typesetting practices.  In the past,(2) (*note ms
Footnotes-Footnote-2::) an 'FL' register was used for the line length in
footnotes; however, setting this register at document initialization
time had no effect on the footnote line length in multi-column
arrangements.(3)  (*note ms Footnotes-Footnote-3::)

   'FR' should be used in preference to the old 'FL' register in
contemporary documents.  The footnote line length is effectively
computed as 'column-width * \*[FR]'.  If an absolute footnote line
length is required, recall that arithmetic expressions in 'roff' input
are evaluated strictly from left to right, with no operator precedence
(parentheses are honored).

     .ds FR 0+3i \" Set footnote line length to 3 inches.

   (1) "Portable Document Format Publishing with GNU Troff",
'pdfmark.ms' in the 'groff' distribution, uses this technique.

   (2) Unix Version 7 'ms', its descendants, and GNU 'ms' prior to
'groff' version 1.23.0

   (3) You could reset it after each call to '.1C', '.2C', or '.MC'.

4.6.5.11 Language and localization
..................................

'groff' 'ms' provides several strings that you can customize for your
own purposes, or redefine to adapt the macro package to languages other
than English.  It is already localized for Czech, German, French,
Italian, and Swedish.  Load the desired localization macro package after
'ms'; see the 'groff_tmac(5)' man page.

     $ groff -ms -mfr bienvenue.ms

   The following strings are available.

 -- String: \*[REFERENCES]
     Contains the string printed at the beginning of a references
     (bibliography) page produced with GNU 'refer(1)'.  The default is
     'References'.

 -- String: \*[ABSTRACT]
     Contains the string printed at the beginning of the abstract.  The
     default is '\f[I]ABSTRACT\f[]'; it includes font selection escape
     sequences to set the word in italics.

 -- String: \*[TOC]
     Contains the string printed at the beginning of the table of
     contents.  The default is 'Table of Contents'.

 -- String: \*[MONTH1]
 -- String: \*[MONTH2]
 -- String: \*[MONTH3]
 -- String: \*[MONTH4]
 -- String: \*[MONTH5]
 -- String: \*[MONTH6]
 -- String: \*[MONTH7]
 -- String: \*[MONTH8]
 -- String: \*[MONTH9]
 -- String: \*[MONTH10]
 -- String: \*[MONTH11]
 -- String: \*[MONTH12]
     Contain the full names of the calendar months.  The defaults are in
     English: 'January', 'February', and so on.

4.6.6 Page layout
-----------------

'ms''s default page layout arranges text in a single column with the
page number between hyphens centered in a header on each page except the
first, and produces no footers.  You can customize this arrangement.

4.6.6.1 Headers and footers
...........................

There are multiple ways to produce headers and footers.  One is to
define the strings 'LH', 'CH', and 'RH' to set the left, center, and
right headers, respectively; and 'LF', 'CF', and 'RF' to set the left,
center, and right footers.  This approach suffices for documents that do
not distinguish odd- and even-numbered pages.

   Another method is to call macros that set headers or footers for odd-
or even-numbered pages.  Each such macro takes a delimited argument
separating the left, center, and right header or footer texts from each
other.  You can replace the neutral apostrophes (''') shown below with
any character not appearing in the header or footer text.  These macros
are Berkeley extensions.

 -- Macro: .OH 'left'center'right'
 -- Macro: .EH 'left'center'right'
 -- Macro: .OF 'left'center'right'
 -- Macro: .EF 'left'center'right'
     The 'OH' and 'EH' macros define headers for odd- (recto) and
     even-numbered (verso) pages, respectively; the 'OF' and 'EF' macros
     define footers for them.

   With either method, a percent sign '%' in header or footer text is
replaced by the current page number.  By default, 'ms' places no header
on a page numbered "1" (regardless of its number format).

 -- Macro: .P1
     Typeset the header even on page 1.  To be effective, this macro
     must be called before the header trap is sprung on any page
     numbered "1"; in practice, unless your page numbering is unusual,
     this means that you should call it early, before 'TL' or any
     heading or paragraphing macro.  This is a Berkeley extension.

   For even greater flexibility, 'ms' is designed to permit the
redefinition of the macros that are called when the 'groff' traps that
ordinarily cause the headers and footers to be output are sprung.  'PT'
("page trap") is called by 'ms' when the header is to be written, and
'BT' ("bottom trap") when the footer is to be.  The 'groff' page
location trap that 'ms' sets up to format the header also calls the
(normally undefined) 'HD' macro after 'PT'; you can define 'HD' if you
need additional processing after setting the header (for example, to
draw a line below it).  The 'HD' hook is a Berkeley extension.  Any such
macros you (re)define must implement any desired specialization for
odd-, even-, or first numbered pages.

4.6.6.2 Tab stops
.................

Use the 'ta' request to define tab stops as needed.  *Note Tabs and
Fields::.

 -- Macro: .TA
     Reset the tab stops to the 'ms' default (every 5 ens).  Redefine
     this macro to create a different set of default tab stops.

4.6.6.3 Margins
...............

Control margins using the registers summarized in "Margin settings" in
*note ms Document Control Settings:: above.  There is no setting for the
right margin; the combination of page offset '\n[PO]' and line length
'\n[LL]' determines it.

4.6.6.4 Multiple columns
........................

'ms' can set text in as many columns as reasonably fit on the page.  The
following macros force a page break if a multi-column layout is active
when they are called.  The 'MINGW' register stores the default minimum
gutter width; it is a GNU extension.  When multiple columns are in use,
keeps and the 'HORPHANS' and 'PORPHANS' registers work with respect to
column breaks instead of page breaks.

 -- Macro: .1C
     Arrange page text in a single column (the default).

 -- Macro: .2C
     Arrange page text in two columns.

 -- Macro: .MC [column-width [gutter-width]]
     Arrange page text in multiple columns.  If you specify no
     arguments, it is equivalent to the '2C' macro.  Otherwise,
     COLUMN-WIDTH is the width of each column and GUTTER-WIDTH is the
     minimum distance between columns.

4.6.6.5 Creating a table of contents
....................................

Because 'roff' formatters process their input in a single pass, material
on page 50, for example, cannot influence what appears on page 1--this
poses a challenge for a table of contents at its traditional location in
front matter, if you wish to avoid manually maintaining it.  'ms'
enables the collection of material to be presented in the table of
contents as it appears, saving its page number along with it, and then
emitting the collected contents on demand toward the end of the
document.  The table of contents can then be resequenced to its desired
location by physically rearranging the pages of a printed document, or
as part of post-processing--with a 'sed(1)' script to reorder the pages
in 'troff''s output, with 'pdfjam(1)', or with 'gropdf(1)''s
'.pdfswitchtopage' feature, for example.

   Define an entry to appear in the table of contents by bracketing its
text between calls to the 'XS' and 'XE' macros.  A typical application
is to call them immediately after 'NH' or 'SH' and repeat the heading
text within them.  The 'XA' macro, used within '.XS'/'.XE' pairs,
supplements an entry--for instance, when it requires multiple output
lines, whether because a heading is too long to fit or because style
dictates that page numbers not be repeated.  You may wish to indent the
text thus wrapped to correspond to its heading depth; this can be done
in the entry text by prefixing it with tabs or horizontal motion escape
sequences, or by providing a second argument to the 'XA' macro.  'XS'
and 'XA' automatically associate the page number where they are called
with the text following them, but they accept arguments to override this
behavior.  At the end of the document, call 'TC' or 'PX' to emit the
table of contents; 'TC' resets the page number to 'i' (Roman numeral
one), and then calls 'PX'.  All of these macros are Berkeley extensions.

 -- Macro: .XS [page-number]
 -- Macro: .XA [page-number [indentation]]
 -- Macro: .XE
     Begin, supplement, and end a table of contents entry.  Each entry
     is associated with PAGE-NUMBER (otherwise the current page number);
     a PAGE-NUMBER of 'no' prevents a leader and page number from being
     emitted for that entry.  Use of 'XA' within 'XS'/'XE' is optional;
     it can be repeated.  If INDENTATION is present, a supplemental
     entry is indented by that amount; ens are assumed if no unit is
     indicated.  Text on input lines between 'XS' and 'XE' is stored for
     later recall by 'PX'.

 -- Macro: .PX [no]
     Switch to single-column layout.  Unless 'no' is specified, center
     and interpolate the 'TOC' string in bold and two points larger than
     the body text.  Emit the table of contents entries.

 -- Macro: .TC [no]
     Set the page number to 1, the page number format to lowercase Roman
     numerals, and call 'PX' (with a 'no' argument, if present).

   Here's an example of typical 'ms' table of contents preparation.  We
employ horizontal escape sequences '\h' to indent the entries by
sectioning depth.

     .NH 1
     Introduction
     .XS
     Introduction
     .XE
     ...
     .NH 2
     Methodology
     .XS
     \h'2n'Methodology
     .XA
     \h'4n'Fassbinder's Approach
     \h'4n'Kahiu's Approach
     .XE
     ...
     .NH 1
     Findings
     .XS
     Findings
     .XE
     ...
     .TC

   The remaining features in this subsubsection are GNU extensions.
'groff' 'ms' obviates the need to repeat heading text after 'XS' calls.
Call 'XN' and 'XH' after 'NH' and 'SH', respectively.

 -- Macro: .XN heading-text
 -- Macro: .XH depth heading-text
     Format HEADING-TEXT and create a corresponding table of contents
     entry.  'XN' computes the indentation from the depth of the
     preceding 'NH' call; 'XH' requires a DEPTH argument to do so.

   'groff' 'ms' encourages customization of table of contents entry
production.

 -- Macro: .XN-REPLACEMENT heading-text
 -- Macro: .XH-REPLACEMENT depth heading-text
     These hook macros implement 'XN' and 'XH', respectively.  They call
     'XN-INIT' and pass their HEADING-TEXT arguments to 'XH-UPDATE-TOC'.

 -- Macro: .XN-INIT
 -- Macro: .XH-UPDATE-TOC depth heading-text
     The 'XN-INIT' hook macro does nothing by default.  'XH-UPDATE-TOC'
     brackets HEADING-TEXT with 'XS' and 'XE' calls, indenting it by 2
     ens per level of DEPTH beyond the first.

   We could therefore produce a table of contents similar to that in the
previous example with fewer macro calls.  (The difference is that this
input follows the "Approach" entries with leaders and page numbers.)

     .NH 1
     .XN Introduction
     ...
     .NH 2
     .XN Methodology
     .XH 3 "Fassbinder's Approach"
     .XH 3 "Kahiu's Approach"
     ...
     .NH 1
     .XN Findings
     ...

   To get the section number of the numbered headings into the table of
contents entries, we might define 'XN-REPLACEMENT' as follows.  (We
obtain the heading depth from 'groff' 'ms''s internal register 'nh*hl'.)

     .de XN-REPLACEMENT
     .XN-INIT
     .XH-UPDATE-TOC \\n[nh*hl] \\$@
     \&\\*[SN] \\$*
     ..

   You can change the style of the leader that bridges each table of
contents entry with its page number; define the 'TC-LEADER' special
character by using the 'char' request.  A typical leader combines the
dot glyph '.' with a horizontal motion escape sequence to spread the
dots.  The width of the page number field is stored in the 'TC-MARGIN'
register.

4.6.7 Differences from AT&T 'ms'
--------------------------------

The 'groff' 'ms' macros are an independent reimplementation, using no
AT&T code.  Since they take advantage of the extended features of
'groff', they cannot be used with AT&T 'troff'.  'groff' 'ms' supports
features described above as Berkeley and Tenth Edition Research Unix
extensions, and adds several of its own.

   * The internals of 'groff' 'ms' differ from the internals of AT&T
     'ms'.  Documents that depend upon implementation details of AT&T
     'ms' may not format properly with 'groff' 'ms'.  Such details
     include macros whose function was not documented in the AT&T 'ms'
     manual.(1)  (*note Differences from AT&T ms-Footnote-1::)

   * The error-handling policy of 'groff' 'ms' is to detect and report
     errors, rather than to ignore them silently.

   * Tenth Edition Research Unix supported 'P1'/'P2' macros to bracket
     code examples; 'groff' 'ms' does not.

   * 'groff' 'ms' does not work in GNU 'troff''s AT&T compatibility
     mode.  If loaded when that mode is enabled, it aborts processing
     with a diagnostic message.

   * Multiple line spacing is not supported.  Use a larger vertical
     spacing instead.

   * 'groff' 'ms' uses the same header and footer defaults in both
     'nroff' and 'troff' modes as AT&T 'ms' does in 'troff' mode; AT&T's
     default in 'nroff' mode is to put the date, in U.S. traditional
     format (e.g., "January 1, 2021"), in the center footer (the 'CF'
     string).

   * Many 'groff' 'ms' macros, including those for paragraphs, headings,
     and displays, cause a reset of paragraph rendering parameters, and
     may change the indentation; they do so not by incrementing or
     decrementing it, but by setting it absolutely.  This can cause
     problems for documents that define additional macros of their own
     that try to manipulate indentation.  Use the 'ms' 'RS' and 'RE'
     macros instead of the 'in' request.

   * AT&T 'ms' interpreted the values of the registers 'PS' and 'VS' in
     points, and did not support the use of scaling units with them.
     'groff' 'ms' interprets values of the registers 'PS', 'VS', 'FPS',
     and 'FVS' equal to or larger than 1,000 (one thousand) as decimal
     fractions multiplied by 1,000.(2)  (*note Differences from AT&T
     ms-Footnote-2::) This threshold makes use of a scaling unit with
     these parameters practical for high-resolution devices while
     preserving backward compatibility.  It also permits expression of
     non-integral type sizes.  For example, 'groff -rPS=10.5p' at the
     shell prompt is equivalent to placing '.nr PS 10.5p' at the
     beginning of the document.

   * AT&T 'ms''s 'AU' macro supported arguments used with some document
     types; 'groff' 'ms' does not.

   * Right-aligned displays are available.  The AT&T 'ms' manual
     observes that "it is tempting to assume that '.DS R' will right
     adjust lines, but it doesn't work".  In 'groff' 'ms', it does.

   * To make 'groff' 'ms' use the default page offset (which also
     specifies the left margin), the 'PO' register must stay undefined
     until the first 'ms' macro is called.

     This implies that '\n[PO]' should not be used early in the
     document, unless it is changed also: accessing an undefined
     register automatically defines it.

   * 'groff' 'ms' supports the 'PN' register, but it is not necessary;
     you can access the page number via the usual '%' register and
     invoke the 'af' request to assign a different format to it if
     desired.(3)  (*note Differences from AT&T ms-Footnote-3::)

   * The AT&T 'ms' manual documents registers 'CW' and 'GW' as setting
     the default column width and "intercolumn gap", respectively, and
     which applied when 'MC' was called with fewer than two arguments.
     'groff' 'ms' instead treats 'MC' without arguments as synonymous
     with '2C'; there is thus no occasion for a default column width
     register.  Further, the 'MINGW' register and the second argument to
     'MC' specify a _minimum_ space between columns, not the fixed
     gutter width of AT&T 'ms'.

   * The AT&T 'ms' manual did not document the 'QI' register; Berkeley
     and 'groff' 'ms' do.

 -- Register: \n[GS]
     The register 'GS' is set to 1 by the 'groff' 'ms' macros, but is
     not used by the AT&T 'ms' package.  Documents that need to
     determine whether they are being formatted with 'groff' 'ms' or
     another implementation should test this register.

   (1) 'Typing Documents on the UNIX System: Using the -ms Macros with
Troff and Nroff', M. E. Lesk, Bell Laboratories, 1978

   (2) Register values are converted to and stored as basic units.
*Note Measurements::.

   (3) If you redefine the 'ms' 'PT' macro and desire special treatment
of certain page numbers (like '1'), you may need to handle a non-Arabic
page number format, as 'groff' 'ms''s 'PT' does; see the macro package
source.  'groff' 'ms' aliases the 'PN' register to '%'.

4.6.7.1 Unix Version 7 'ms' macros not implemented by 'groff' 'ms'
..................................................................

Several macros described in the Unix Version 7 'ms' documentation are
unimplemented by 'groff' 'ms' because they are specific to the
requirements of documents produced internally by Bell Laboratories, some
of which also require a glyph for the Bell System logo that 'groff' does
not support.  These macros implemented several document type formats
('EG', 'IM', 'MF', 'MR', 'TM', 'TR'), were meaningful only in
conjunction with the use of certain document types ('AT', 'CS', 'CT',
'OK', 'SG'), stored the postal addresses of Bell Labs sites ('HO', 'IH',
'MH', 'PY', 'WH'), or lacked a stable definition over time ('UX').  To
compatibly render historical 'ms' documents using these macros, we
advise your documents to invoke the 'rm' request to remove any such
macros it uses and then define replacements with an authentically
typeset original at hand.(1)  (*note Missing Unix Version 7 ms
Macros-Footnote-1::) For informal purposes, a simple definition of 'UX'
should maintain the readability of the document's substance.

     .rm UX
     .ds UX Unix\"

   (1) The removal beforehand is necessary because 'groff' 'ms' aliases
these macros to a diagnostic macro, and you want to redefine the aliased
name, not its target.

4.6.8 Legacy Features
---------------------

'groff' 'ms' retains some legacy features solely to support formatting
of historical documents; contemporary ones should not use them because
they can render poorly.  See the 'groff_char(7)' man page.

AT&T accent mark strings
........................

AT&T 'ms' defined accent mark strings as follows.

 -- String: \*[']
     Apply acute accent to subsequent glyph.

 -- String: \*[`]
     Apply grave accent to subsequent glyph.

 -- String: \*[:]
     Apply dieresis (umlaut) to subsequent glyph.

 -- String: \*[^]
     Apply circumflex accent to subsequent glyph.

 -- String: \*[~]
     Apply tilde accent to subsequent glyph.

 -- String: \*[C]
     Apply caron to subsequent glyph.

 -- String: \*[,]
     Apply cedilla to subsequent glyph.

Berkeley accent mark and glyph strings
......................................

Berkeley 'ms' offered an 'AM' macro; calling it redefined the AT&T
accent mark strings (except for '\*C'), applied them to the _preceding_
glyph, and defined additional strings, some for spacing glyphs.

 -- Macro: .AM
     Enable alternative accent mark and glyph-producing strings.

 -- String: \*[']
     Apply acute accent to preceding glyph.

 -- String: \*[`]
     Apply grave accent to preceding glyph.

 -- String: \*[:]
     Apply dieresis (umlaut) to preceding glyph.

 -- String: \*[^]
     Apply circumflex accent to preceding glyph.

 -- String: \*[~]
     Apply tilde accent to preceding glyph.

 -- String: \*[,]
     Apply cedilla to preceding glyph.

 -- String: \*[/]
     Apply stroke (slash) to preceding glyph.

 -- String: \*[v]
     Apply caron to preceding glyph.

 -- String: \*[_]
     Apply macron to preceding glyph.

 -- String: \*[.]
     Apply underdot to preceding glyph.

 -- String: \*[o]
     Apply ring accent to preceding glyph.

 -- String: \*[?]
     Interpolate inverted question mark.

 -- String: \*[!]
     Interpolate inverted exclamation mark.

 -- String: \*[8]
     Interpolate small letter sharp s.

 -- String: \*[q]
     Interpolate small letter o with hook accent (ogonek).

 -- String: \*[3]
     Interpolate small letter yogh.

 -- String: \*[d-]
     Interpolate small letter eth.

 -- String: \*[D-]
     Interpolate capital letter eth.

 -- String: \*[th]
     Interpolate small letter thorn.

 -- String: \*[Th]
     Interpolate capital letter thorn.

 -- String: \*[ae]
     Interpolate small � ligature.

 -- String: \*[Ae]
     Interpolate capital � ligature.

 -- String: \*[oe]
     Interpolate small oe ligature.

 -- String: \*[OE]
     Interpolate capital OE ligature.

4.6.9 Naming Conventions
------------------------

The following conventions are used for names of macros, strings, and
registers.  External names available to documents that use the 'groff'
'ms' macros contain only uppercase letters and digits.

   Internally, the macros are divided into modules.  Conventions for
identifier names are as follows.

   * Names used only within one module are of the form MODULE'*'NAME.

   * Names used outside the module in which they are defined are of the
     form MODULE'@'NAME.

   * Names associated with a particular environment are of the form
     ENVIRONMENT':'NAME; these are used only within the 'par' module.

   * NAME does not have a module prefix.

   * Constructed names used to implement arrays are of the form
     ARRAY'!'INDEX.

   Thus the 'groff' 'ms' macros reserve the following names.

   * Names containing the characters '*', '@', and ':'.

   * Names containing only uppercase letters and digits.

5 GNU 'troff' Reference
***********************

This chapter covers _all_ of the facilities of the GNU 'troff'
formatting engine.  Users of macro packages may skip it if not
interested in details.

5.1 Text
========

AT&T 'troff' was designed to take input as it would be composed on a
typewriter, including the teletypewriters used as early computer
terminals, and relieve the user drafting a document of concern with
details like line length, hyphenation breaking, and the achievement of
straight margins.  Early in its development, the program gained the
ability to prepare output for a phototypesetter; a document could then
be prepared for output to either a teletypewriter, a phototypesetter, or
both.  GNU 'troff' continues this tradition of permitting an author to
compose a single master version of a document which can then be rendered
for a variety of output formats or devices.

   'roff' input files contain text interspersed with instructions to
control the formatter.  Even in the absence of such instructions, GNU
'troff' still processes its input in several ways, by filling,
hyphenating, breaking, and adjusting it, and supplementing it with
inter-sentence space.

5.1.1 Filling
-------------

When GNU 'troff' starts up, it obtains information about the device for
which it is preparing output.(1)  (*note Filling-Footnote-1::) An
essential property is the length of the output line, such as "6.5
inches".

   GNU 'troff' interprets plain text files employing the Unix
line-ending convention.  It reads input a character at a time,
collecting words as it goes, and fits as many words together on an
output line as it can--this is known as "filling".  To GNU 'troff', a
"word" is any sequence of one or more characters that aren't spaces or
newlines.  The exceptions separate words.(2)  (*note
Filling-Footnote-2::) To disable filling, see *note Manipulating Filling
and Adjustment::.

     It is a truth universally acknowledged
     that a single man in possession of a
     good fortune must be in want of a wife.
         => It is a truth universally acknowledged that a
         => single man in possession of a good fortune must
         => be in want of a wife.

   (1) *Note Device and Font Description Files::.

   (2) Tabs and leaders also separate words.  Escape sequences can
function as word characters, word separators, or neither--the last
simply have no effect on GNU 'troff''s idea of whether an input
character is within a word.  We'll discuss all of these in due course.

5.1.2 Sentences
---------------

A passionate debate has raged for decades among writers of the English
language over whether more space should appear between adjacent
sentences than between words within a sentence, and if so, how much, and
what other circumstances should influence this spacing.(1)  (*note
Sentences-Footnote-1::) GNU 'troff' follows the example of AT&T 'troff';
it attempts to detect the boundaries between sentences, and supplies
additional inter-sentence space between them.

     Hello, world!
     Welcome to groff.
         => Hello, world!  Welcome to groff.

   GNU 'troff' flags certain characters (normally '!', '?', and '.') as
potentially ending a sentence.  When GNU 'troff' encounters one of these
"end-of-sentence characters" at the end of an input line, or one of them
is followed by two (unescaped) spaces on the same input line, it appends
an inter-word space followed by an inter-sentence space in the output.

     R. Harper subscribes to a maxim of P. T. Barnum.
         => R. Harper subscribes to a maxim of P. T. Barnum.

   In the above example, inter-sentence space is not added after 'P.' or
'T.' because the periods do not occur at the end of an input line, nor
are they followed by two or more spaces.  Let's imagine that we've heard
something about defamation from Mr. Harper's attorney, recast the
sentence, and reflowed it in our text editor.

     I submit that R. Harper subscribes to a maxim of P. T.
     Barnum.
         => I submit that R. Harper subscribes to a maxim of
         => P. T.  Barnum.

   "Barnum" doesn't begin a sentence!  What to do?  Let us meet our
first "escape sequence", a series of input characters that give
instructions to GNU 'troff' instead of being used to construct output
device glyphs.(2)  (*note Sentences-Footnote-2::) An escape sequence
begins with the backslash character '\' by default, an uncommon
character in natural language text, and is _always_ followed by at least
one other character, hence the term "sequence".

   The dummy character escape sequence '\&' can be used after an
end-of-sentence character to defeat end-of-sentence detection on a
per-instance basis.  We can therefore rewrite our input more
defensively.

     I submit that R.\& Harper subscribes to a maxim of P.\&
     T.\& Barnum.
         => I submit that R. Harper subscribes to a maxim of
         => P. T. Barnum.

   Adding text caused our input to wrap; now, we don't need '\&' after
'T.' but we do after 'P.'.  Consistent use of the escape sequence
ensures that potential sentence boundaries are robust to editing
activities.  Further advice along these lines will follow in *note Input
Conventions::.

   Normally, the occurrence of a visible non-end-of-sentence character
(as opposed to a space or tab) immediately after an end-of-sentence
character cancels detection of the end of a sentence.  For example, it
would be incorrect for GNU 'troff' to infer the end of a sentence after
the dot in '3.14159'.  However, several characters are treated
_transparently_ after the occurrence of an end-of-sentence character.
That is, GNU 'troff' does not cancel end-of-sentence detection when it
processes them.  This is because such characters are often used as
footnote markers or to close quotations and parentheticals.  The default
set is '"', ''', ')', ']', '*', '\[dg]', '\[dd]', '\[rq]', and '\[cq]'.
The last four are examples of "special characters", escape sequences
whose purpose is to obtain glyphs that are not easily typed at the
keyboard, or which have special meaning to GNU 'troff' (like '\'
itself).(3)  (*note Sentences-Footnote-3::)

     \[lq]The idea that the poor should have leisure has always
     been shocking to the rich.\[rq]
     (Bertrand Russell, 1935)
         => "The idea that the poor should have
         => leisure has always been shocking to
         => the rich."  (Bertrand Russell, 1935)

   The sets of characters that potentially end sentences or are
transparent to sentence endings are configurable.  See the 'cflags'
request in *note Using Symbols::.  To change the additional
inter-sentence space amount--even to remove it entirely--see *note
Manipulating Filling and Adjustment::.

   (1) A well-researched jeremiad appreciated by 'groff' contributors on
both sides of the sentence-spacing debate can be found at
<https://web.archive.org/web/20171217060354/http://www.heracliteanriver.com/?p=324>.

   (2) This statement oversimplifies; there are escape sequences whose
purpose is precisely to produce glyphs on the output device, and input
characters that _aren't_ part of escape sequences can undergo a great
deal of processing before getting to the output.

   (3) The mnemonics for the special characters shown here are "dagger",
"double dagger", "right (double) quote", and "closing (single) quote".
See the 'groff_char(7)' man page.

5.1.3 Hyphenation
-----------------

When an output line is nearly full, it is uncommon for the next word
collected from the input to exactly fill it--typically, there is room
left over only for part of the next word.  The process of splitting a
word so that it appears partially on one line (with a hyphen to indicate
to the reader that the word has been broken) with its remainder on the
next is "hyphenation".  Hyphenation points can be manually specified;
GNU 'troff' also uses a hyphenation algorithm and language-specific
pattern files (based on those used in TeX) to decide which words can be
hyphenated and where.

   Hyphenation does not always occur even when the hyphenation rules for
a word allow it; it can be disabled, and when not disabled there are
several parameters that can prevent it in certain circumstances.  *Note
Manipulating Hyphenation::.

5.1.4 Breaking
--------------

Once an output line is full, the next word (or remainder of a hyphenated
one) is placed on a different output line; this is called a "break".  In
this manual and in 'roff' discussions generally, a "break" if not
further qualified always refers to the termination of an output line.
When the formatter is filling text, it introduces breaks automatically
to keep output lines from exceeding the configured line length.  After
an automatic break, GNU 'troff' adjusts the line if applicable (see
below), and then resumes collecting and filling text on the next output
line.

   Sometimes, a line cannot be broken automatically.  This usually does
not happen with natural language text unless the output line length has
been manipulated to be extremely short, but it can with specialized text
like program source code.  We can use 'perl' at the shell prompt to
contrive an example of failure to break the line.  We also employ the
'-z' option to suppress normal output.

     $ perl -e 'print "#" x 80, "\n";' | nroff -z
         error-> warning: cannot break line

   The remedy for these cases is to tell GNU 'troff' where the line may
be broken without hyphens.  This is done with the non-printing break
point escape sequence '\:'; see *note Manipulating Hyphenation::.

   What if the document author wants to stop filling lines temporarily,
for instance to start a new paragraph?  There are several solutions.  A
blank input line not only causes a break, but by default it also outputs
a one-line vertical space (effectively a blank output line).  This
behavior can be modified; see *note Blank Line Traps::.  Macro packages
may discourage or disable the blank line method of paragraphing in favor
of their own macros.

   A line that begins with one or more spaces causes a break.  The
spaces are output at the beginning of the next line without being
_adjusted_ (see below); however, this behavior can be modified (*note
Leading Space Traps::).  Again, macro packages may provide other methods
of producing indented paragraphs.  Trailing spaces on text lines are
discarded.(1)  (*note Breaking-Footnote-1::)

   What if the file ends before enough words have been collected to fill
an output line?  Or the output line is exactly full but not yet broken,
and there is no more input?  GNU 'troff' interprets the end of input as
a break.  Certain requests also cause breaks, implicitly or explicitly.
This is discussed in *note Manipulating Filling and Adjustment::.

   (1) "Text lines" are defined in *note Requests and Macros::.

5.1.5 Adjustment
----------------

After GNU 'troff' performs an automatic break, it may then "adjust" the
line, widening inter-word spaces until the text reaches the right
margin.  Extra spaces between words are preserved.  Leading and trailing
spaces are handled as noted above.  Text can be aligned to the left or
right margin only, or centered; see *note Manipulating Filling and
Adjustment::.

5.1.6 Tabs and Leaders
----------------------

GNU 'troff' translates input horizontal tab characters ("tabs") and
<Control+A> characters ("leaders") into movements to the next tab stop.
Tabs simply move to the next tab stop; leaders place enough periods to
fill the space.  Tab stops are by default located every half inch
measured from the drawing position corresponding to the beginning of the
input line; see *note Page Geometry::.  Tabs and leaders do not cause
breaks and therefore do not interrupt filling.  Below, we use arrows ->
and bullets * to indicate input tabs and leaders, respectively.

     1
     -> 2 -> 3 * 4
     -> * 5
     => 1         2       3.......4         ........5

   Tabs and leaders lend themselves to table construction.(1)  (*note
Tabs and Leaders-Footnote-1::) The tab and leader glyphs can be
configured, and further facilities for sophisticated table composition
are available; see *note Tabs and Fields::.  There are many details to
track when using such low-level features, so most users turn to the
'tbl(1)' preprocessor to lay out tables.

   (1) "Tab" is short for "tabulation", revealing the term's origin as a
spacing mechanism for table arrangement.

5.1.7 Requests and Macros
-------------------------

We have now encountered almost all of the syntax there is in the 'roff'
language, with an exception already noted in passing.  A "request" is an
instruction to the formatter that occurs after a "control character",
which is recognized at the beginning of an input line.  The regular
control character is a dot ('.').  Its counterpart, the "no-break
control character", a neutral apostrophe ('''), suppresses the break
that is implied by some requests.  These characters were chosen because
it is uncommon for lines of text in natural languages to begin with
them.  If you require a formatted period or apostrophe (closing single
quotation mark) where GNU 'troff' is expecting a control character,
prefix the dot or neutral apostrophe with the dummy character escape
sequence, '\&'.

   An input line beginning with a control character is called a "control
line".  Every line of input that is not a control line is a "text
line".(1)  (*note Requests and Macros-Footnote-1::)

   Requests often take "arguments", words (separated from the request
name and each other by spaces) that specify details of the action GNU
'troff' is expected to perform.  If a request is meaningless without
arguments, it is typically ignored.

   GNU 'troff''s requests and escape sequences comprise the control
language of the formatter.  Of key importance are the requests that
define macros.  Macros are invoked like requests, enabling the request
repertoire to be extended or overridden.(2)  (*note Requests and
Macros-Footnote-2::)

   A "macro" can be thought of as an abbreviation you can define for a
collection of control and text lines.  When the macro is "called" by
giving its name after a control character, it is replaced with what it
stands for.  The process of textual replacement is known as
"interpolation".(3)  (*note Requests and Macros-Footnote-3::)
Interpolations are handled as soon as they are recognized, and once
performed, a 'roff' formatter scans the replacement for further
requests, macro calls, and escape sequences.

   In 'roff' systems, the 'de' request defines a macro.(4)  (*note
Requests and Macros-Footnote-4::)

     .de DATE
     2020-11-14
     ..

The foregoing input produces no output by itself; all we have done is
store some information.  Observe the pair of dots that ends the macro
definition.  This is a default; you can specify your own terminator for
the macro definition as the second argument to the 'de' request.

     .de NAME ENDNAME
     Heywood Jabuzzoff
     .ENDNAME

   In fact, the ending marker is itself the name of a macro to be
called, or a request to be invoked, if it is defined at the time its
control line is read.

     .de END
     Big Rip
     ..
     .de START END
     Big Bang
     .END
     .START
         => Big Rip Big Bang

In the foregoing example, "Big Rip" printed before "Big Bang" because
its macro was _called_ first.  Consider what would happen if we dropped
'END' from the '.de START' line and added '..' after '.END'.  Would the
order change?

   Let us consider a more elaborate example.

     .de DATE
     2020-10-05
     ..
     .
     .de BOSS
     D.\& Kruger,
     J.\& Peterman
     ..
     .
     .de NOTICE
     Approved:
     .DATE
     by
     .BOSS
     ..
     .
     Insert tedious regulatory compliance paragraph here.

     .NOTICE

     Insert tedious liability disclaimer paragraph here.

     .NOTICE
         => Insert tedious regulatory compliance paragraph here.
         =>
         => Approved: 2020-10-05 by D. Kruger, J. Peterman
         =>
         => Insert tedious liability disclaimer paragraph here.
         =>
         => Approved: 2020-10-05 by D. Kruger, J. Peterman

The above document started with a series of control lines.  Three macros
were defined, with a 'de' request declaring each macro's name, and the
"body" of the macro starting on the next line and continuing until a
line with two dots ''..'' marked its end.  The text proper began only
after the macros were defined; this is a common pattern.  Only the
'NOTICE' macro was called "directly" by the document; 'DATE' and 'BOSS'
were called only by 'NOTICE' itself.  Escape sequences were used in
'BOSS', two levels of macro interpolation deep.

   The advantage in typing and maintenance economy may not be obvious
from such a short example, but imagine a much longer document with
dozens of such paragraphs, each requiring a notice of managerial
approval.  Consider what must happen if you are in charge of generating
a new version of such a document with a different date, for a different
boss.  With well-chosen macros, you only have to change each datum in
one place.

   In practice, we would probably use strings (*note Strings::) instead
of macros for such simple interpolations; what is important here is to
glimpse the potential of macros and the power of recursive
interpolation.

   We could have defined 'DATE' and 'BOSS' in the opposite order;
perhaps less obviously, we could also have defined them _after_
'NOTICE'.  "Forward references" like this are acceptable because the
body of a macro definition is not (completely) interpreted, but stored
instead (*note Copy Mode::).  While a macro is being defined (or
appended to), requests are not interpreted and macros not interpolated,
whereas some commonly used escape sequences _are_ interpreted.  'roff'
systems also support recursive macro calls, as long as you have a way to
break the recursion (*note Conditionals and Loops::).  Maintainable
'roff' documents tend to arrange macro definitions to minimize forward
references.

   (1) The '\<RET>' escape sequence can alter how an input line is
classified; see *note Line Continuation::.

   (2) Argument handling in macros is more flexible but also more
complex.  *Note Calling Macros::.

   (3) Some escape sequences undergo interpolation as well.

   (4) GNU 'troff' offers additional ones.  *Note Writing Macros::.

5.1.8 Macro Packages
--------------------

Macro definitions can be collected into "macro files", 'roff' input
files designed to produce no output themselves but instead ease the
preparation of other 'roff' documents.  There is no syntactical
difference between a macro file and any other 'roff' document; only its
purpose distinguishes it.  When a macro file is installed at a standard
location and suitable for use by a general audience, it is often termed
a "macro package".(1)  (*note Macro Packages-Footnote-1::) Macro
packages can be loaded by supplying the '-m' option to GNU 'troff' or a
'groff' front end.  Alternatively, a document requiring a macro package
can load it with the 'mso' ("macro source") request.

   (1) Macro files and packages frequently define registers and strings
as well.

5.1.9 Input Encodings
---------------------

The 'groff' command's '-k' option calls the 'preconv' preprocessor to
perform input character encoding conversions.  Input to the GNU 'troff'
formatter itself, on the other hand, must be in one of two encodings it
can recognize.

'cp1047'
     The code page 1047 input encoding works only on EBCDIC platforms
     (and conversely, the other input encodings don't work with EBCDIC);
     the file 'cp1047.tmac' is loaded at startup.

'latin1'
     ISO Latin-1, an encoding for Western European languages, is the
     default input encoding on non-EBCDIC platforms; the file
     'latin1.tmac' is loaded at startup.

Any document that is encoded in ISO 646:1991 (a descendant of USAS
X3.4-1968 or "US-ASCII"), or, equivalently, uses only code points from
the "C0 Controls" and "Basic Latin" parts of the Unicode character set
is also a valid ISO Latin-1 document; the standards are interchangeable
in their first 128 code points.(1)  (*note Input Encodings-Footnote-1::)

   Other encodings are supported by means of macro packages.

'latin2'
     To use ISO Latin-2, an encoding for Central and Eastern European
     languages, invoke '.mso latin2.tmac' at the beginning of your
     document or supply '-mlatin2' as a command-line argument to
     'groff'.

'latin5'
     To use ISO Latin-5, an encoding for the Turkish language, invoke
     '.mso latin5.tmac' at the beginning of your document or supply
     '-mlatin5' as a command-line argument to 'groff'.

'latin9'
     ISO Latin-9 succeeds Latin-1; it includes a Euro sign and better
     glyph coverage for French.  To use this encoding, invoke
     '.mso latin9.tmac' at the beginning of your document or supply
     '-mlatin9' as a command-line argument to 'groff'.

   Some characters from an input encoding may not be available with a
particular output driver, or their glyphs may not have representation in
the font used.  For terminal devices, fallbacks are defined, like 'EUR'
for the Euro sign and '(C)' for the copyright sign.  For typesetter
devices, you may need to "mount" fonts that support glyphs required by
the document.  *Note Font Positions::.

   Because a Euro glyph was not historically defined in PostScript
fonts, 'groff' comes with a font called 'freeeuro.pfa' that provides the
Euro in several styles.  Standard PostScript fonts contain the glyphs
from Latin-5 and Latin-9 that Latin-1 lacks, so these encodings are
supported for the 'ps' and 'pdf' output devices as 'groff' ships, while
Latin-2 is not.

   Unicode supports characters from all other input encodings; the
'utf8' output driver for terminals therefore does as well.  The DVI
output driver supports the Latin-2 and Latin-9 encodings if the
command-line option '-mec' is used as well.  (2)  (*note Input
Encodings-Footnote-2::)

   (1) The _semantics_ of certain punctuation code points have gotten
stricter with the successive standards, a cause of some frustration
among man page writers; see the 'groff_char(7)' man page.

   (2) The DVI output device defaults to using the Computer Modern (CM)
fonts; 'ec.tmac' loads the EC fonts instead, which provide Euro '\[Eu]'
and per mille '\[%0]' glyphs.

5.1.10 Input Conventions
------------------------

Since GNU 'troff' fills text automatically, it is common practice in the
'roff' language to avoid visual composition of text in input files: the
esthetic appeal of the formatted output is what matters.  Therefore,
'roff' input should be arranged such that it is easy for authors and
maintainers to compose and develop the document, understand the syntax
of 'roff' requests, macro calls, and preprocessor languages used, and
predict the behavior of the formatter.  Several traditions have accrued
in service of these goals.

   * Follow sentence endings in the input with newlines to ease their
     recognition (*note Sentences::).  It is frequently convenient to
     end text lines after colons and semicolons as well, as these
     typically precede independent clauses.  Consider doing so after
     commas; they often occur in lists that become easy to scan when
     itemized by line, or constitute supplements to the sentence that
     are added, deleted, or updated to clarify it.  Parenthetical and
     quoted phrases are also good candidates for placement on text lines
     by themselves.

   * Set your text editor's line length to 72 characters or fewer.(1)
     (*note Input Conventions-Footnote-1::) This limit, combined with
     the previous item of advice, makes it less common that an input
     line will wrap in your text editor, and thus will help you perceive
     excessively long constructions in your text.  Recall that natural
     languages originate in speech, not writing, and that punctuation is
     correlated with pauses for breathing and changes in prosody.

   * Use '\&' after '!', '?', and '.' if they are followed by space,
     tab, or newline characters and don't end a sentence.

   * In filled text lines, use '\&' before '.' and ''' if they are
     preceded by space, so that reflowing the input doesn't turn them
     into control lines.

   * Do not use spaces to perform indentation or align columns of a
     table.  Leading spaces are reliable when text is not being filled.

   * Comment your document.  It is never too soon to apply comments to
     record information of use to future document maintainers (including
     your future self).  We thus introduce another escape sequence,
     '\"', which causes GNU 'troff' to ignore the remainder of the input
     line.

   * Use the empty request--a control character followed immediately by
     a newline--to visually manage separation of material in input
     files.  Many of the 'groff' project's own documents use an empty
     request between sentences, after macro definitions, and where a
     break is expected, and two empty requests between paragraphs or
     other requests or macro calls that will introduce vertical space
     into the document.

     You can combine the empty request with the comment escape sequence
     to include whole-line comments in your document, and even "comment
     out" sections of it.

   We conclude this section with an example sufficiently long to
illustrate most of the above suggestions in practice.  For the purpose
of fitting the example between the margins of this manual with the font
used for its typeset version, we have shortened the input line length to
56 columns.  As before, an arrow -> indicates a tab character.

     .\"   nroff this_file.roff | less
     .\"   groff -T ps this_file.roff > this_file.ps
     ->The theory of relativity is intimately connected with
     the theory of space and time.
     .
     I shall therefore begin with a brief investigation of
     the origin of our ideas of space and time,
     although in doing so I know that I introduce a
     controversial subject.  \" remainder of paragraph elided
     .
     .

     ->The experiences of an individual appear to us arranged
     in a series of events;
     in this series the single events which we remember
     appear to be ordered according to the criterion of
     \[lq]earlier\[rq] and \[lq]later\[rq], \" punct swapped
     which cannot be analysed further.
     .
     There exists,
     therefore,
     for the individual,
     an I-time,
     or subjective time.
     .
     This itself is not measurable.
     .
     I can,
     indeed,
     associate numbers with the events,
     in such a way that the greater number is associated with
     the later event than with an earlier one;
     but the nature of this association may be quite
     arbitrary.
     .
     This association I can define by means of a clock by
     comparing the order of events furnished by the clock
     with the order of a given series of events.
     .
     We understand by a clock something which provides a
     series of events which can be counted,
     and which has other properties of which we shall speak
     later.
     .\" Albert Einstein, _The Meaning of Relativity_, 1922

   (1) Emacs: 'fill-column: 72'; Vim: 'textwidth=72'

5.2 Page Geometry
=================

'roff' systems format text under certain assumptions about the size of
the output medium, or page.  For the formatter to correctly break a line
it is filling, it must know the line length, which it derives from the
page width (*note Line Layout::).  For it to decide whether to write an
output line to the current page or wait until the next one, it must know
the page length (*note Page Layout::).

   A device's "resolution" converts practical units like inches or
centimeters to "basic units", a convenient length measure for the output
device or file format.  The formatter and output driver use basic units
to reckon page measurements.  The device description file defines its
resolution and page dimensions (*note DESC File Format::).

   A "page" is a two-dimensional structure upon which a 'roff' system
imposes a rectangular coordinate system with its upper left corner as
the origin.  Coordinate values are in basic units and increase down and
to the right.  Useful ones are therefore always positive and within
numeric ranges corresponding to the page boundaries.

   While the formatter (and, later, output driver) is processing a page,
it keeps track of its "drawing position", which is the location at which
the next glyph will be written, from which the next motion will be
measured, or where a geometric object will commence rendering.
Notionally, glyphs are drawn from the text baseline upward and to the
right.(1)  (*note Page Geometry-Footnote-1::) The "text baseline" is a
(usually invisible) line upon which the glyphs of a typeface are
aligned.  A glyph therefore "starts" at its bottom-left corner.  If
drawn at the origin, a typical letter glyph would lie partially or
wholly off the page, depending on whether, like "g", it features a
descender below the baseline.

   Such a situation is nearly always undesirable.  It is furthermore
conventional not to write or draw at the extreme edges of the page.
Therefore the initial drawing position of a 'roff' formatter is not at
the origin, but below and to the right of it.  This rightward shift from
the left edge is known as the "page offset".(2)  (*note Page
Geometry-Footnote-2::) The downward shift leaves room for a text output
line.

   Text is arranged on a one-dimensional lattice of text baselines from
the top to the bottom of the page.  "Vertical spacing" is the distance
between adjacent text baselines.  Typographic tradition sets this
quantity to 120% of the type size.  The initial drawing position is one
unit of vertical spacing below the page top.  Typographers term this
unit a vee.

   Vertical spacing has an impact on page-breaking decisions.
Generally, when a break occurs, the formatter moves the drawing position
to the next text baseline automatically.  If the formatter were already
writing to the last line that would fit on the page, advancing by one
vee would place the next text baseline off the page.  Rather than let
that happen, 'roff' formatters instruct the output driver to eject the
page, start a new one, and again set the drawing position to one vee
below the page top; this is a "page break".

   When the last line of input text corresponds to the last output line
that fits on the page, the break caused by the end of input will also
break the page, producing a useless blank one.  Macro packages keep
users from having to confront this difficulty by setting "traps" (*note
Traps::); moreover, all but the simplest page layouts tend to have
headers and footers, or at least bear vertical margins larger than one
vee.

   (1) 'groff' does not yet support right-to-left scripts.

   (2) 'groff''s terminal output devices have page offsets of zero.

5.3 Measurements
================

The formatter sometimes requires the input of numeric parameters to
specify measurements.  These are specified as integers or decimal
fractions with an optional "scaling unit" suffixed.  A scaling unit is a
letter that immediately follows the last digit of a number.  Digits
after the decimal point are optional.  Measurement expressions include
'10.5p', '11i', and '3.c'.

   Measurements are scaled by the scaling unit and stored internally
(with any fractional part discarded) in basic units.  The device
resolution can therefore be obtained by storing a value of '1i' to a
register.  The only constraint on the basic unit is that it is at least
as small as any other unit.

'u'
     Basic unit.

'i'
     Inch; defined as 2.54 centimeters.

'c'
     Centimeter; a centimeter is about 0.3937 inches.

'p'
     Point; a typesetter's unit used for measuring type size.  There are
     72 points to an inch.

'P'
     Pica; another typesetter's unit.  There are 6 picas to an inch and
     12 points to a pica.

's'
'z'
     *Note Using Fractional Type Sizes::, for a discussion of these
     units.

'f'
     GNU 'troff' defines this unit to scale decimal fractions in the
     interval [0, 1] to 16-bit unsigned integers.  It multiplies a
     quantity by 65,536.  *Note Colors::, for usage.

   The magnitudes of other scaling units depend on the text formatting
parameters in effect.  These are useful when specifying measurements
that need to scale with the typeface or vertical spacing.

'm'
     Em; an em is equal to the current type size in points.  It is named
     thus because it is approximately the width of the letter 'M'.

'n'
     En; an en is one-half em.

'v'
     Vee; recall *note Page Geometry::.

'M'
     Hundredth of an em.

5.3.1 Motion Quanta
-------------------

An output device's basic unit 'u' is not necessarily its smallest
addressable length; 'u' can be smaller to avoid problems with integer
roundoff.  The minimum distances that a device can work with in the
horizontal and vertical directions are termed its "motion quanta".
Measurements are rounded to applicable motion quanta.  Half-quantum
fractions round toward zero.

 -- Register: \n[.H]
 -- Register: \n[.V]
     These read-only registers interpolate the horizontal and vertical
     motion quanta, respectively, of the output device in basic units.

   For example, we might draw short baseline rules on a terminal device
as follows.  *Note Drawing Geometric Objects::.

     .tm \n[.H]
         error-> 24
     .nf
     \l'36u' 36u
     \l'37u' 37u
         => _ 36u
         => __ 37u

5.3.2 Default Units
-------------------

A general-purpose register (one created or updated with the 'nr'
request; see *note Registers::) is implicitly dimensionless, or reckoned
in basic units if interpreted in a measurement context.  But it is
convenient for many requests and escape sequences to infer a scaling
unit for an argument if none is specified.  An explicit scaling unit
(not after a closing parenthesis) can override an undesirable default.
Effectively, the default unit is suffixed to the expression if a scaling
unit is not already present.  GNU 'troff''s use of integer arithmetic
should also be kept in mind (*note Numeric Expressions::).

   The 'll' request interprets its argument in ems by default.  Consider
several attempts to set a line length of 3.5 inches when the type size
is 10 points on a terminal device with a resolution of 240 basic units
and horizontal motion quantum of 24.  Some expressions become zero; the
request clamps them to that quantum.

     .ll 3.5i      \" 3.5i (= 840u)
     .ll 7/2       \" 7u/2u -> 3u -> 3m -> 0, clamped to 24u
     .ll (7 / 2)u  \" 7u/2u -> as above
     .ll 7/2i      \" 7u/2i -> 7u/480u -> 0 -> as above
     .ll 7i/2      \" 7i/2u -> 1680u/2m -> 1680u/24u -> 35u
     .ll 7i/2u     \" 3.5i (= 840u)

The safest way to specify measurements is to attach a scaling unit.  To
multiply or divide by a dimensionless quantity, use 'u' as its scaling
unit.

5.4 Numeric Expressions
=======================

A "numeric expression" evaluates to an integer: it can be as simple as a
literal '0' or it can be a complex sequence of register and string
interpolations interleaved with measurements and operators.

   GNU 'troff' provides a set of mathematical and logical operators
familiar to programmers--as well as some unusual ones--but supports only
integer arithmetic.(1)  (*note Numeric Expressions-Footnote-1::) The
internal data type used for computing results is usually a 32-bit signed
integer, which suffices to represent magnitudes within a range of �2
billion.(2)  (*note Numeric Expressions-Footnote-2::)

   Arithmetic infix operators perform a function on the numeric
expressions to their left and right; they are '+' (addition), '-'
(subtraction), '*' (multiplication), '/' (truncating division), and '%'
(modulus).  "Truncating division" rounds to the integer nearer to zero,
no matter how large the fractional portion.  Overflow and division (or
modulus) by zero are errors and abort evaluation of a numeric
expression.

   Arithmetic unary operators operate on the numeric expression to their
right; they are '-' (negation) and '+' (assertion--for completeness; it
does nothing).  The unary minus must often be used with parentheses to
avoid confusion with the decrementation operator, discussed below.

   Observe the rounding behavior and effect of negative operands on the
modulus and truncating division operators.

     .nr T 199/100
     .nr U 5/2
     .nr V (-5)/2
     .nr W 5/-2
     .nr X 5%2
     .nr Y (-5)%2
     .nr Z 5%-2
     T=\n[T] U=\n[U] V=\n[V] W=\n[W] X=\n[X] Y=\n[Y] Z=\n[Z]
         => T=1 U=2 V=-2 W=-2 X=1 Y=-1 Z=1

The sign of the modulus of operands of mixed signs is determined by the
sign of the first.  Division and modulus operators satisfy the following
property: given a dividend A and a divisor B, a quotient Q formed by '(a
/ b)' and a remainder R by '(a % b)', then qb + r = a.

   GNU 'troff''s scaling operator, used with parentheses as '(C;E)',
evaluates a numeric expression E using C as the default scaling unit.
If C is omitted, scaling units are ignored in the evaluation of E.  This
operator can save typing by avoiding the attachment of scaling units to
every operand out of caution.  Your macros can select a sensible default
unit in case the user neglects to supply one.

     .\" Indent by amount given in first argument; assume ens.
     .de Indent
     .  in (n;\\$1)
     ..

Without the scaling operator, the foregoing macro would, if called with
a unitless argument, cause indentation by the 'in' request's default
scaling unit (ems).  The result would be twice as much indentation as
expected.

   GNU 'troff' also provides a pair of operators to compute the extrema
of two operands: '>?' (maximum) and '<?' (minimum).

     .nr slots 5
     .nr candidates 3
     .nr salaries (\n[slots] <? \n[candidates])
     Looks like we'll end up paying \n[salaries] salaries.
         => Looks like we'll end up paying 3 salaries.

   Comparison operators comprise '<' (less than), '>' (greater than),
'<=' (less than or equal), '>=' (greater than or equal), and '='
(equal).  '==' is a synonym for '='.  When evaluated, a comparison is
replaced with '0' if it is false and '1' if true.  In the 'roff'
language, positive values are true, others false.

   We can operate on truth values with the logical operators '&'
(logical conjunction or "and") and ':' (logical disjunction or "or").
They evaluate as comparison operators do.

   A logical complementation ("not") operator, '!', works only within
'if', 'ie', and 'while' requests.  Furthermore, '!' is recognized only
at the beginning of a numeric expression not contained by another
numeric expression.  In other words, it must be the "outermost"
operator.  Including it elsewhere in the expression produces a warning
in the 'number' category (*note Warnings::), and its expression
evaluates false.  This unfortunate limitation maintains compatibility
with AT&T 'troff'.  Test a numeric expression for falsity by comparing
it to a false value.(3)  (*note Numeric Expressions-Footnote-3::)

     .nr X 1
     .nr Y 0
     .\" This does not work as expected.
     .if (\n[X])&(!\n[Y]) .nop A: X is true, Y is false
     .
     .\" Use this construct instead.
     .if (\n[X])&(\n[Y]<=0) .nop B: X is true, Y is false
         error-> warning: expected numeric expression, got '!'
         => B: X is true, Y is false

   The 'roff' language has no operator precedence: expressions are
evaluated strictly from left to right, in contrast to schoolhouse
arithmetic.  Use parentheses '(' ')' to impose a desired precedence upon
subexpressions.

     .nr X 3+5*4
     .nr Y (3+5)*4
     .nr Z 3+(5*4)
     X=\n[X] Y=\n[Y] Z=\n[Z]
         => X=32 Y=32 Z=23

   For many requests and escape sequences that cause motion on the page,
the unary operators '+' and '-' work differently when leading a numeric
expression.  They then indicate a motion relative to the drawing
position: positive is down in vertical contexts, right in horizontal
ones.

   '+' and '-' are also treated differently by the following requests
and escape sequences: 'bp', 'in', 'll', 'lt', 'nm', 'nr', 'pl', 'pn',
'po', 'ps', 'pvs', 'rt', 'ti', '\H', '\R', and '\s'.  Here, leading plus
and minus signs serve as incrementation and decrementation operators,
respectively.  To negate an expression, subtract it from zero or include
the unary minus in parentheses with its argument.  *Note Setting
Registers::, for examples.

   A leading '|' operator indicates a motion relative not to the drawing
position but to a boundary.  For horizontal motions, the measurement
specifies a distance relative to a drawing position corresponding to the
beginning of the _input_ line.  By default, tab stops reckon movements
in this way.  Most escape sequences do not; '|' tells them to do so.

     Mind the \h'1.2i'gap.
     .br
     Mind the \h'|1.2i'gap.
     .br
     Mind the
     \h'|1.2i'gap.
         => Mind the             gap.
         => Mind the    gap.
         => Mind the             gap.

   One use of this feature is to define macros whose scope is limited to
the output they format.

     .\" underline word $1 with trailing punctuation $2
     .de Underline
     .  nop \\$1\l'|0\[ul]'\\$2
     ..
     Typographical emphasis is best used
     .Underline sparingly .

In the above example, '|0' specifies a negative motion from the current
position (at the end of the argument just emitted, '\$1') to the
beginning of the input line.  Thus, the '\l' escape sequence in this
case draws a line from right to left.  A macro call occurs at the
beginning of an input line;(4) (*note Numeric Expressions-Footnote-4::)
if the '|' operator were omitted, then the underline would be drawn at
zero distance from the current position, producing device-dependent, and
likely undesirable, results.  On the 'ps' output device, it underlines
the period.

   For vertical motions, the '|' operator specifies a distance from the
first text baseline on the page or in the current diversion,(5) (*note
Numeric Expressions-Footnote-5::) using the current vertical spacing.

     A
     .br
     B \Z'C'\v'|0'D
         => A D
         => B C

   In the foregoing example, we've used the '\Z' escape sequence (*note
Page Motions::) to restore the drawing position after formatting 'C',
then moved vertically to the first text baseline on the page.

 -- Escape sequence: \B'anything'
     Interpolate 1 if ANYTHING is a valid numeric expression, and 0
     otherwise.  The delimiter need not be a neutral apostrophe; see
     *note Delimiters::.

   You might use '\B' along with the 'if' request to filter out invalid
macro or string arguments.  *Note Conditionals and Loops::.

     .\" Indent by amount given in first argument; assume ens.
     .de Indent
     .  if \B'\\$1' .in (n;\\$1)
     ..

   A register interpolated as an operand in a numeric expression must
have an Arabic format; luckily, this is the default.  *Note Assigning
Register Formats::.

   Because spaces separate arguments to requests, spaces are not allowed
in numeric expressions unless the (sub)expression containing them is
surrounded by parentheses.  *Note Invoking Requests::, and *note
Conditionals and Loops::.

     .nf
     .nr a 1+2 + 2+1
     \na
         error-> expected numeric expression, got a space
         => 3
     .nr a 1+(2 + 2)+1
     \na
         => 6

   The 'nr' request (*note Setting Registers::) expects its second and
optional third arguments to be numeric expressions; a bare '+' does not
qualify, so our first attempt got a warning.

   (1) Provision is made for interpreting and reporting decimal
fractions in certain cases.

   (2) If that's not enough, see the 'groff_tmac(5)' man page for the
'62bit.tmac' macro package.

   (3) *Note Conditionals and Loops::.

   (4) Control structure syntax creates an exception to this rule, but
is designed to remain useful: recalling our example, '.if 1 .Underline
this' would underline only "this", precisely.  *Note Conditionals and
Loops::.

   (5) *Note Diversions::.

5.5 Identifiers
===============

An "identifier" labels a GNU 'troff' datum such as a register, name
(macro, string, or diversion), typeface, color, special character,
character class, environment, or stream.  Valid identifiers consist of
one or more ordinary characters.  An ordinary character is an input
character that is not the escape character, a leader, tab, newline, or
invalid as GNU 'troff' input.

   Invalid input characters are a subset of control characters (from the
sets "C0 Controls" and "C1 Controls" as Unicode describes them).  When
GNU 'troff' encounters one in an identifier, it produces a warning in
category 'input' (*note Warnings::).  They are removed during
interpretation: an identifier 'foo', followed by an invalid character
and then 'bar', is processed as 'foobar'.

   On a machine using the ISO 646, 8859, or 10646 character encodings,
invalid input characters are '0x00', '0x08', '0x0B', '0x0D'-'0x1F', and
'0x80'-'0x9F'.  On an EBCDIC host, they are '0x00'-'0x01', '0x08',
'0x09', '0x0B', '0x0D'-'0x14', '0x17'-'0x1F', and '0x30'-'0x3F'.(1)
(*note Identifiers-Footnote-1::) Some of these code points are used by
GNU 'troff' internally, making it non-trivial to extend the program to
accept UTF-8 or other encodings that use characters from these
ranges.(2)  (*note Identifiers-Footnote-2::)

   Thus, the identifiers 'br', 'PP', 'end-list', 'ref*normal-print',
'|', '@_', and '!"#$%'()*+,-./' are all valid.  Discretion should be
exercised to prevent confusion.  Identifiers starting with '(' or '['
require care.

     .nr x 9
     .nr y 1
     .nr (x 2
     .nr [y 3
     .nr sum1 (\n(x + \n[y])
         error-> a space character is not allowed in an escape
         error->   sequence parameter
     A:2+3=\n[sum1]
     .nr sum2 (\n((x + \n[[y])
     B:2+3=\n[sum2]
     .nr sum3 (\n[(x] + \n([y)
     C:2+3=\n[sum3]
         => A:2+3=1 B:2+3=5 C:2+3=5

An identifier with a closing bracket (']') in its name can't be accessed
with bracket-form escape sequences that expect an identifier as a
parameter.  For example, '\[foo]]' accesses the glyph 'foo', followed by
']' in whatever the surrounding context is, whereas '\C'foo]'' formats a
glyph named 'foo]'.  Similarly, the identifier '(' can't be interpolated
_except_ with bracket forms.

   If you begin a macro, string, or diversion name with either of the
characters '[' or ']', you foreclose use of the 'grefer' preprocessor,
which recognizes '.[' and '.]' as bibliographic reference delimiters.

 -- Escape sequence: \A'anything'
     Interpolate 1 if ANYTHING is a valid identifier, and 0 otherwise.
     The delimiter need not be a neutral apostrophe; see *note
     Delimiters::.  Because invalid input characters are removed (see
     above), invalid identifiers are empty or contain spaces, tabs, or
     newlines.

     You can employ '\A' to validate a macro argument before using it to
     construct another escape sequence or identifier.

          .\" usage: .init-coordinate-pair name val1 val2
          .\" Create a coordinate pair where name!x=val1 and
          .\" name!y=val2.
          .de init-coordinate-pair
          .  if \A'\\$1' \{\
          .    if \B'\\$2' .nr \\$1!x \\$2
          .    if \B'\\$3' .nr \\$1!y \\$3
          .  \}
          ..
          .init-coordinate-pair center 5 10
          The center is at (\n[center!x], \n[center!y]).
          .init-coordinate-pair "poi->nt" trash garbage \" ignored
          .init-coordinate-pair point trash garbage \" ignored
              => The center is at (5, 10).

     In this example, we also validated the numeric arguments; the
     registers 'point!x' and 'point!y' remain undefined.  *Note Numeric
     Expressions:: for the '\B' escape sequence.

   How GNU 'troff' handles the interpretation of an undefined identifier
depends on the context.  There is no way to invoke an undefined request;
such syntax is interpreted as a macro call instead.  If the identifier
is interpreted as a string, macro, or diversion, GNU 'troff' emits a
warning in category 'mac', defines it as empty, and interpolates
nothing.  If the identifier is interpreted as a register, GNU 'troff'
emits a warning in category 'reg', initializes it to zero, and
interpolates that value.  *Note Warnings::, *note Interpolating
Registers::, and *note Strings::.  Attempting to use an undefined
typeface, special character, color, character class, environment, or
stream generally provokes an error diagnostic.

   Identifiers for requests, macros, strings, and diversions share one
name space; special characters and character classes another.  No other
object types do.

     .de xxx
     .  nop foo
     ..
     .di xxx
     bar
     .br
     .di
     .
     .xxx
         => bar

The foregoing example shows that GNU 'troff' reuses the identifier
'xxx', changing it from a macro to a diversion.  No warning is emitted,
and the previous contents of 'xxx' are lost.

   (1) Historically, control characters like ASCII STX, ETX, and BEL
(<Control+B>, <Control+C>, and <Control+G>) have been observed in 'roff'
documents, particularly in macro packages employing them as delimiters
with the output comparison operator to try to avoid collisions with the
content of arbitrary user-supplied parameters (*note Operators in
Conditionals::).  We discourage this expedient; in GNU 'troff' it is
unnecessary (outside of compatibility mode) because delimited arguments
are parsed at a different input level than the surrounding context.
*Note Implementation Differences::.

   (2) Consider what happens when a C1 control '0x80'-'0x9F' is
necessary as a continuation byte in a UTF-8 sequence.

5.6 Formatter Instructions
==========================

To support documents that require more than filling, automatic line
breaking and hyphenation, adjustment, and supplemental inter-sentence
space, the 'roff' language offers two means of embedding instructions to
the formatter.

   One is a "request", which begins with a control character and takes
up the remainder of the input line.  Requests often perform relatively
large-scale operations such as setting the page length, breaking the
line, or starting a new page.  They also conduct internal operations
like defining macros.

   The other is an "escape sequence", which begins with the escape
character and can be embedded anywhere in the input, even in arguments
to requests and other escape sequences.  Escape sequences interpolate
special characters, strings, or registers, and handle comparatively
minor formatting tasks like sub- and superscripting.

   Some operations, such as font selection and type size alteration, are
available via both requests and escape sequences.

5.6.1 Control Characters
------------------------

The mechanism of using 'roff''s control characters to invoke requests
and call macros was introduced in *note Requests and Macros::.  Control
characters are recognized only at the beginning of an input line, or at
the beginning of the branch of a control structure request; see *note
Conditionals and Loops::.

   A few requests cause a break implicitly; use the no-break control
character to prevent the break.  Break suppression is its sole
behavioral distinction.  Employing the no-break control character to
invoke requests that don't cause breaks is harmless but poor style.
*Note Manipulating Filling and Adjustment::.

   The control '.' and no-break control ''' characters can each be
changed to any ordinary character(1) (*note Control
Characters-Footnote-1::) with the 'cc' and 'c2' requests, respectively.

 -- Request: .cc [o]
     Recognize the ordinary character O as the control character.  If O
     is absent or invalid, the default control character '.' is
     selected.  The identity of the control character is associated with
     the environment (*note Environments::).

 -- Request: .c2 [o]
     Recognize the ordinary character O as the no-break control
     character.  If O is absent or invalid, the default no-break control
     character ''' is selected.  The identity of the no-break control
     character is associated with the environment (*note
     Environments::).

   When writing a macro, you might wish to know which control character
was used to call it.

 -- Register: \n[.br]
     This read-only register interpolates 1 if the currently executing
     macro was called using the normal control character and 0
     otherwise.  If a macro is interpolated as a string, the '.br'
     register's value is inherited from the context of the string
     interpolation.  *Note Strings::.

     Use this register to reliably intercept requests that imply breaks.

          .als bp*orig bp
          .de bp
          .  ie \\n[.br] .bp*orig
          .  el          'bp*orig
          ..

     Testing the '.br' register outside of a macro definition makes no
     sense.

   (1) Recall *note Identifiers::.

5.6.2 Invoking Requests
-----------------------

A control character is optionally followed by tabs and/or spaces and
then an identifier naming a request or macro.  The invocation of an
unrecognized request is interpreted as a macro call.  Defining a macro
with the same name as a request replaces the request.  Deleting a
request name with the 'rm' request makes it unavailable.  The 'als'
request can alias requests, permitting them to be wrapped or
non-destructively replaced.  *Note Strings::.

   There is no inherent limit on argument length or quantity.  Most
requests take one or more arguments, and ignore any they do not expect.
A request may be separated from its arguments by tabs or spaces, but
only spaces can separate an argument from its successor.  Only one
between arguments is necessary; any excess is ignored.  GNU 'troff' does
not allow tabs for argument separation.(1)  (*note Invoking
Requests-Footnote-1::)

   Generally, a space _within_ a request argument is not relevant, not
meaningful, or is supported by bespoke provisions, as with the 'tl'
request's delimiters (*note Page Layout::).  Some requests, like 'ds',
interpret the remainder of the control line as a single argument.  *Note
Strings::.

   Spaces and tabs immediately after a control character are ignored.
Commonly, authors structure the source of documents or macro files with
them.

     .de center
     .  if \\n[.br] \
     .    br
     .  ce \\$1
     ..
     .
     .
     .de right-align
     .->if \\n[.br] \
     .->->br
     .->rj \\$1
     ..

   If you assign an empty blank line trap, you can separate macro
definitions (or any input lines) with blank lines.

     .de do-nothing
     ..
     .blm do-nothing  \" activate blank line trap

     .de center
     .  if \\n[.br] \
     .    br
     .  ce \\$1
     ..


     .de right-align
     .->if \\n[.br] \
     .->->br
     .->rj \\$1
     ..

     .blm             \" deactivate blank line trap

   *Note Blank Line Traps::.

   (1) In compatibility mode, a space is not necessary after a request
or macro name of two characters' length.  Also, Plan 9 'troff' allows
tabs to separate arguments.

5.6.3 Calling Macros
--------------------

If a macro of the desired name does not exist when called, it is
created, assigned an empty definition, and a warning in category 'mac'
is emitted.  Calling an undefined macro _does_ end a macro definition
naming it as its end macro (*note Writing Macros::).

   To embed spaces _within_ a macro argument, enclose the argument in
neutral double quotes '"'.  Horizontal motion escape sequences are
sometimes a better choice for arguments to be formatted as text.

   Consider calls to a hypothetical section heading macro 'uh'.

     .uh The Mouse Problem
     .uh "The Mouse Problem"
     .uh The\~Mouse\~Problem
     .uh The\ Mouse\ Problem

The first line calls 'uh' with three arguments: 'The', 'Mouse', and
'Problem'.  The remainder call the 'uh' macro with one argument, 'The
Mouse Problem'.  The last solution, using escaped spaces, can be found
in documents prepared for AT&T 'troff'.  It can cause surprise when text
is adjusted, because '\<SP>' inserts a _fixed-width_, non-breaking
space.  GNU 'troff''s '\~' escape sequence inserts an adjustable,
non-breaking space.(1)  (*note Calling Macros-Footnote-1::)

   The foregoing raises the question of how to embed neutral double
quotes or backslashes in macro arguments when _those_ characters are
desired as literals.  In GNU 'troff', the special character escape
sequence '\[rs]' produces a backslash and '\[dq]' a neutral double
quote.

   In GNU 'troff''s AT&T compatibility mode, these characters remain
available as '\(rs' and '\(dq', respectively.  AT&T 'troff' did not
consistently define these special characters, but its descendants can be
made to support them.  *Note Device and Font Description Files::.

   If even that is not feasible, options remain.  To obtain a literal
escape character in a macro argument, you can simply type it if you
change or disable the escape character first.  *Note Using Escape
Sequences::.  Otherwise, you must escape the escape character repeatedly
to a context-dependent extent.  *Note Copy Mode::.

   For the (neutral) double quote, you have recourse to an obscure
syntactical feature of AT&T 'troff'.  Because a double quote can begin a
macro argument, the formatter keeps track of whether the current
argument was started thus, and doesn't require a space after the double
quote that ends it.(2)  (*note Calling Macros-Footnote-2::) In the
argument list to a macro, a double quote that _isn't_ preceded by a
space _doesn't_ start a macro argument.  If not preceded by a double
quote that began an argument, this double quote becomes part of the
argument.  Furthermore, within a quoted argument, a pair of adjacent
double quotes becomes a literal double quote.

     .de eq
     .  tm arg1:\\$1 arg2:\\$2 arg3:\\$3
     .  tm arg4:\\$4 arg5:\\$5 arg6:\\$6
     .. \" 4 backslashes on the next line
     .eq a" "b c" "de"f\\\\g" h""i "j""k"
         error-> arg1:a" arg2:b c arg3:de
         error-> arg4:f\g" arg5:h""i arg6:j"k

   Apart from the complexity of the rules, this traditional solution has
the disadvantage that double quotes don't survive repeated argument
expansion in AT&T 'troff' or GNU 'troff''s compatibility mode.  This can
frustrate efforts to pass such arguments intact through multiple macro
calls.

     .cp 1
     .de eq
     .  tm arg1:\\$1 arg2:\\$2 arg3:\\$3
     .  tm arg4:\\$4 arg5:\\$5 arg6:\\$6
     ..
     .de xe
     .  eq \\$1 \\$2 \\$3 \\$4 \\$5 \\$6
     .. \" 8 backslashes on the next line
     .xe a" "b c" "de"f\\\\\\\\g" h""i "j""k"
         error-> arg1:a" arg2:b arg3:c
         error-> arg4:de arg5:f\g" arg6:h""i

   Outside of compatibility mode, GNU 'troff' doesn't exhibit this
problem because it tracks the nesting depth of interpolations.  *Note
Implementation Differences::.

   (1) '\~' is fairly portable; see *note Other Differences::.

   (2) Strictly, you can neglect to close the last quoted macro
argument, relying on the end of the control line to do so.  We consider
this lethargic practice poor style.

5.6.4 Using Escape Sequences
----------------------------

Whereas requests must occur on control lines, escape sequences can occur
intermixed with text and may appear in arguments to requests, macros,
and other escape sequences.  An escape sequence is introduced by the
escape character, a backslash '\' (but see the 'ec' request below).  The
next character selects the escape's function.

   Escape sequences vary in length.  Some take an argument, and of
those, some have different syntactical forms for a one-character,
two-character, or arbitrary-length argument.  Others accept _only_ an
arbitrary-length argument.  In the former scheme, a one-character
argument follows the function character immediately, an opening
parenthesis '(' introduces a two-character argument (no closing
parenthesis is used), and an argument of arbitrary length is enclosed in
brackets '[]'.  In the latter scheme, the user selects a delimiter
character.  A few escape sequences are idiosyncratic, and support both
of the foregoing conventions ('\s'), designate their own termination
sequence ('\?'), consume input until the next newline ('\!', '\"',
'\#'), or support an additional modifier character ('\s' again, and
'\n').  As with requests, use of some escape sequences in source
documents may interact poorly with a macro package you use; consult its
documentation to learn of "safe" sequences or alternative facilities it
provides to achieve the desired result.

   If an escape character is followed by a character that does not
identify a defined operation, the escape character is ignored (producing
a diagnostic of the 'escape' warning category, which is not enabled by
default) and the following character is processed normally.

     $ groff -Tps -ww
     .nr N 12
     .ds co white
     .ds animal elephant
     I have \fI\nN \*(co \*[animal]s,\f[]
     said \P.\&\~Pseudo Pachyderm.
         error-> warning: escape character ignored before 'P'
         => I have 12 white elephants, said P. Pseudo Pachyderm.

   Escape sequence interpolation is of higher precedence than escape
sequence argument interpretation.  This rule affords flexibility in
using escape sequences to construct parameters to other escape
sequences.

     .ds family C\" Courier
     .ds style I\" oblique
     Choice a typeface \f(\*[family]\*[style]wisely.
         => Choose a typeface wisely.

In the above, the syntax form '\f(' accepts only two characters for an
argument; the example works because the subsequent escape sequences are
interpolated before the selection escape sequence argument is processed,
and strings 'family' and 'style' interpolate one character each.(1)
(*note Using Escape Sequences-Footnote-1::)

   The escape character is nearly always interpreted when encountered;
it is therefore desirable to have a way to interpolate it, disable it,
or change it.

 -- Escape sequence: \e
     Interpolate the escape character.

   The '\[rs]' special character escape sequence formats a backslash
glyph.  In macro and string definitions, the input sequences '\\' and
'\E' defer interpretation of escape sequences.  *Note Copy Mode::.

 -- Request: .eo
     Disable the escape mechanism except in copy mode.  Once this
     request is invoked, no input character is recognized as starting an
     escape sequence in interpretation mode.

 -- Request: .ec [o]
     Recognize the ordinary character O as the escape character.  If O
     is absent or invalid, the default escape character '\' is selected.

   Switching escape sequence interpretation off to define a macro and
back on afterward can obviate the need to double the escape character
within the definition.  *Note Writing Macros::.  This technique is not
available if your macro needs to interpolate values at the time it is
_defined_--but many do not.

     .\" simplified `BR` macro from the man(7) macro package
     .eo
     .de BR
     .  ds result \&
     .  while (\n[.$] >= 2) \{\
     .    as result \fB\$1\fR\$2\"
     .    shift 2
     .  \}
     .  if \n[.$] .as result \fB\$1\"
     \*[result]
     .  rm result
     .  ft R
     ..
     .ec

 -- Request: .ecs
 -- Request: .ecr
     The 'ecs' request stores the escape character for recall with
     'ecr'.  'ecr' sets the escape character to '\' if none has been
     saved.

     Use these requests together to temporarily change the escape
     character.

   Using a different escape character, or disabling it, when calling
macros not under your control will likely cause errors, since GNU
'troff' has no mechanism to "intern" macros--that is, to convert a macro
definition into a form independent of its representation.(2)  (*note
Using Escape Sequences-Footnote-2::) When a macro is called, its
contents are interpreted literally.

   (1) The omission of spaces before the comment escape sequences is
necessary; see *note Strings::.

   (2) TeX does have such a mechanism.

5.6.5 Delimiters
----------------

Some escape sequences that require parameters use delimiters.  The
neutral apostrophe ''' is a popular choice and shown in this document.
The neutral double quote '"' is also commonly seen.  Letters, numerals,
and leaders can be used.  Punctuation characters are likely better
choices, except for those defined as infix operators in numeric
expressions; see below.

     \l'1.5i\[bu]' \" draw 1.5 inches of bullet glyphs

   The following escape sequences don't take arguments and thus are
allowed as delimiters: '\<SP>', '\%', '\|', '\^', '\{', '\}', '\'',
'\`', '\-', '\_', '\!', '\?', '\)', '\/', '\,', '\&', '\:', '\~', '\0',
'\a', '\c', '\d', '\e', '\E', '\p', '\r', '\t', and '\u'.  However,
using them this way is discouraged; they can make the input confusing to
read.

   A few escape sequences, '\A', '\b', '\o', '\w', '\X', and '\Z',
accept a newline as a delimiter.  Newlines that serve as delimiters
continue to be recognized as input line terminators.

     A caf\o
     e\(aa
     in Paris
         => A caf� in Paris

Use of newlines as delimiters in escape sequences is also discouraged.

   Finally, the escape sequences '\D', '\h', '\H', '\l', '\L', '\N',
'\R', '\s', '\S', '\v', and '\x' prohibit many delimiters.

   * the numerals '0'-'9' and the decimal point '.'

   * the (single-character) operators '+-/*%<>=&:()'

   * the space and tab characters

   * any escape sequences other than '\%', '\:', '\{', '\}', '\'', '\`',
     '\-', '\_', '\!', '\/', '\c', '\e', and '\p'

   Delimiter syntax is complex and flexible primarily for historical
reasons; the foregoing restrictions need be kept in mind mainly when
using 'groff' in AT&T compatibility mode.  GNU 'troff' keeps track of
the nesting depth of escape sequence interpolations, so the only
characters you need to avoid using as delimiters are those that appear
in the arguments you input, not any that result from interpolation.
Typically, ''' works fine.  *Note Implementation Differences::.

     $ groff -Tps
     .de Mw
     .  nr wd \w'\\$1'
     .  tm "\\$1" is \\n(wd units wide.
     ..
     .Mw Wet'suwet'en
     .Mw Wet+200i
     .cp 1 \" turn on compatibility mode
     .Mw Wet'suwet'en
     .Mw Wet'
     .Mw Wet+200i
         error-> "Wet'suwet'en" is 54740 units wide.
         error-> "Wet'+200i" is 42610 units wide.
         error-> "Wet'suwet'en" is 15860 units wide.
         error-> "Wet'" is 15860 units wide.
         error-> "Wet'+200i" is 14415860 units wide.

   We see here that in compatibility mode, the part of the argument
after the ''' delimiter escapes from its context and, if nefariously
crafted, influences the computation of the WD register's value in a
surprising way.

5.7 Comments
============

One of the most common forms of escape sequence is the comment.(1)
(*note Comments-Footnote-1::)

 -- Escape sequence: \"
     Start a comment.  Everything up to the next newline is ignored.

     This may sound simple, but it can be tricky to keep the comments
     from interfering with the appearance of the output.  If the escape
     sequence is to the right of some text or a request, that portion of
     the line is ignored, but spaces preceding it are processed normally
     by GNU 'troff'.  This affects only the 'ds' and 'as' requests and
     their variants.

     One possibly irritating idiosyncrasy is that tabs should not be
     used to vertically align comments in the source document.  Tab
     characters are not treated as separators between a request name and
     its first argument, nor between arguments.

     A comment on a line by itself is treated as a blank line, because
     after eliminating the comment, that is all that remains.

          Test
          \" comment
          Test
              => Test
              =>
              => Test

     To avoid this, it is common to combine the empty request with the
     comment escape sequence as '.\"', causing the input line to be
     ignored.

     Another commenting scheme sometimes seen is three consecutive
     single quotes (''''') at the beginning of a line.  This works, but
     GNU 'troff' emits a warning diagnostic (if enabled) about an
     undefined macro (namely '''').

 -- Escape sequence: \#
     Start a comment; everything up to and including the next newline is
     ignored.  This 'groff' extension was introduced to avoid the
     problems described above.

          Test
          \# comment
          Test
              => Test Test

 -- Request: .ig [end]
     Ignore input until, in the current conditional block (if any),(2)
     (*note Comments-Footnote-2::) the macro END is called at the start
     of a control line, or the control line '..' is encountered if END
     is not specified.  'ig' is parsed as if it were a macro definition,
     but its contents are discarded, not stored.(3)  (*note
     Comments-Footnote-3::)

          hand\c
          .de TX
          fasting
          ..
          .ig TX
          This is part of a large block of input that has been
          temporarily(?) commented out.
          We can restore it simply by removing the .ig request and
          the call of its end macro.
          .TX
              => handfasting

   (1) This claim may be more aspirational than descriptive.

   (2) *Note Conditional Blocks::.

   (3) Exception: auto-incrementing registers defined outside the
ignored region _will_ be modified if interpolated with '\n�' inside it.
*Note Auto-increment::.

5.8 Registers
=============

In the 'roff' language, numbers can be stored in "registers".  Many
built-in registers exist, supplying anything from the date to details of
formatting parameters.  You can also define your own.  *Note
Identifiers::, for information on constructing a valid name for a
register.

5.8.1 Setting Registers
-----------------------

Define registers and update their values with the 'nr' request or the
'\R' escape sequence.

 -- Request: .nr ident value
 -- Escape sequence: \R'ident value'
     Set register IDENT to VALUE.  If IDENT doesn't exist, GNU 'troff'
     creates it.  In the '\R' escape sequence, the delimiter need not be
     a neutral apostrophe; see *note Delimiters::.  It also does not
     produce an input token in GNU 'troff'.  *Note Gtroff Internals::.

          .nr a (((17 + (3 * 4))) % 4)
          \n[a]
          .\R'a (((17 + (3 * 4))) % 4)'
          \n[a]
              => 1 1

     (Later, we will discuss additional forms of 'nr' and '\R' that can
     change a register's value after it is dereferenced but before it is
     interpolated.  *Note Auto-increment::.)

     The complete transparency of '\R' can cause surprising effects if
     you use registers like '.k', which get evaluated at the time they
     are accessed.

          .ll 1.6i
          .
          aaa bbb ccc ddd eee fff ggg hhh\R':k \n[.k]'
          .tm :k == \n[:k]
              => :k == 126950
          .
          .br
          .
          aaa bbb ccc ddd eee fff ggg hhh\h'0'\R':k \n[.k]'
          .tm :k == \n[:k]
              => :k == 15000

     If you process this with the PostScript device ('-Tps'), there will
     be a line break eventually after 'ggg' in both input lines.
     However, after processing the space after 'ggg', the partially
     collected line is not overfull yet, so GNU 'troff' continues to
     collect input until it sees the space (or in this case, the
     newline) after 'hhh'.  At this point, the line is longer than the
     line length, and the line gets broken.

     In the first input line, since the '\R' escape sequence leaves no
     traces, the check for the overfull line hasn't been done yet at the
     point where '\R' gets handled, and you get a value for the '.k'
     register that is even greater than the current line length.

     In the second input line, the insertion of '\h'0'' to cause a
     zero-width motion forces GNU 'troff' to check the line length,
     which in turn causes the start of a new output line.  Now '.k'
     returns the expected value.

   'nr' and '\R' each have two additional special forms to increment or
decrement a register.

 -- Request: .nr ident +value
 -- Request: .nr ident -value
 -- Escape sequence: \R'ident +value'
 -- Escape sequence: \R'ident -value'
     Increment (decrement) register IDENT by VALUE.  In the '\R' escape
     sequence, the delimiter need not be a neutral apostrophe; see *note
     Delimiters::.

          .nr a 1
          .nr a +1
          \na
              => 2

     A leading minus sign in VALUE is always interpreted as a
     decrementation operator, not an algebraic sign.  To assign a
     register a negative value or the negated value of another register,
     you can force GNU 'troff' to interpret '-' as a negation or minus,
     rather than decrementation, operator: enclose it with its operand
     in parentheses or subtract it from zero.

          .nr a 7
          .nr b 3
          .nr a -\nb
          \na
              => 4
          .nr a (-\nb)
          \na
              => -3
          .nr a 0-\nb
          \na
              => -3

     If a register's prior value does not exist (the register was
     undefined), an increment or decrement is applied as if to 0.

 -- Request: .rr ident
     Remove register IDENT.  If IDENT doesn't exist, the request is
     ignored.  Technically, only the name is removed; the register's
     contents are still accessible under aliases created with 'aln', if
     any.

 -- Request: .rnn ident1 ident2
     Rename register IDENT1 to IDENT2.  If IDENT1 doesn't exist, the
     request is ignored.  Renaming a built-in register does not
     otherwise alter its properties.

 -- Request: .aln new old
     Create an alias NEW for an existing register OLD, causing the names
     to refer to the same stored object.  If OLD is undefined, a warning
     in category 'reg' is produced and the request is ignored.  *Note
     Warnings::, for information about the enablement and suppression of
     warnings.

     To remove a register alias, invoke 'rr' on its name.  A register's
     contents do not become inaccessible until it has no more names.

5.8.2 Interpolating Registers
-----------------------------

Register contents are interpolated with the '\n' escape sequence.

 -- Escape sequence: \ni
 -- Escape sequence: \n(id
 -- Escape sequence: \n[ident]
     Interpolate register with name IDENT (one-character name I,
     two-character name ID).  '\n' is interpreted even in copy mode
     (*note Copy Mode::).  If the register is undefined, it is created
     and assigned a value of '0', that value is interpolated, and a
     warning in category 'reg' is emitted.  *Note Warnings::, for
     information about the enablement and suppression of warnings.

          .nr a 5
          .nr as \na+\na
          \n(as
              => 10

          .nr a1 5
          .nr ab 6
          .ds str b
          .ds num 1
          \n[a\n[num]]
              => 5
          \n[a\*[str]]
              => 6

5.8.3 Auto-increment
--------------------

Registers can also be incremented or decremented by a configured amount
at the time they are interpolated.  The value of the increment is
specified with a third argument to the 'nr' request, and a special
interpolation syntax is used to alter and then retrieve the register's
value.  Together, these features are called "auto-increment".(1)  (*note
Auto-increment-Footnote-1::)

 -- Request: .nr ident value incr
     Set register IDENT to VALUE and its auto-incrementation amount to
     to INCR.  The '\R' escape sequence doesn't support an INCR
     argument.

   Auto-incrementation is not _completely_ automatic; the '\n' escape
sequence in its basic form never alters the value of a register.  To
apply auto-incrementation to a register, interpolate it with '\n�'.

 -- Escape sequence: \n+i
 -- Escape sequence: \n-i
 -- Escape sequence: \n+(id
 -- Escape sequence: \n-(id
 -- Escape sequence: \n+[ident]
 -- Escape sequence: \n-[ident]
     Increment or decrement IDENT (one-character name I, two-character
     name ID) by the register's auto-incrementation value and then
     interpolate the new register value.  If IDENT has no
     auto-incrementation value, interpolate as with '\n'.

     .nr a 0 1
     .nr xx 0 5
     .nr foo 0 -2
     \n+a, \n+a, \n+a, \n+a, \n+a
     .br
     \n-(xx, \n-(xx, \n-(xx, \n-(xx, \n-(xx
     .br
     \n+[foo], \n+[foo], \n+[foo], \n+[foo], \n+[foo]
         => 1, 2, 3, 4, 5
         => -5, -10, -15, -20, -25
         => -2, -4, -6, -8, -10

   To change the increment value without changing the value of a
register, assign the register's value to itself by interpolating it, and
specify the desired increment normally.  Apply an increment of '0' to
disable auto-incrementation of the register.

   (1) A negative auto-increment can be considered an "auto-decrement".

5.8.4 Assigning Register Formats
--------------------------------

A writable register's value can be interpolated in several number
formats.  By default, conventional Arabic numerals are used.  Other
formats see use in sectioning and outlining schemes and alternative page
numbering arrangements.

 -- Request: .af reg fmt
     Use number format FMT when interpolating register REG.  Valid
     number formats are as follows.

     '0...'
          Arabic numerals 0, 1, 2, and so on.  Any decimal digit is
          equivalent to '0'; the formatter merely counts the digits
          specified.  Multiple Arabic numerals in FMT cause
          interpolations to be zero-padded on the left if necessary to
          at least as many digits as specified (interpolations never
          truncate a register value).  A register with format '00'
          interpolates values 1, 2, 3 as '01', '02', '03'.  The default
          format for all writable registers is '0'.

     'I'
          Uppercase Roman numerals: 0, I, II, III, IV, ...

     'i'
          Lowercase Roman numerals: 0, i, ii, iii, iv, ...

     'A'
          Uppercase letters: 0, A, B, C, ..., Z, AA, AB, ...

     'a'
          Lowercase letters: 0, a, b, c, ..., z, aa, ab, ...

     Omitting FMT causes a warning in category 'missing'.  *Note
     Warnings::, for information about the enablement and suppression of
     warnings.  Specifying an unrecognized format is an error.

     Zero values are interpolated as '0' in non-Arabic formats.
     Negative quantities are prefixed with '-' irrespective of format.
     In Arabic formats, the sign supplements the field width.  If REG
     doesn't exist, it is created with a zero value.

          .nr a 10
          .af a 0           \" the default format
          \na,
          .af a I
          \na,
          .af a 321
          .nr a (-\na)
          \na,
          .af a a
          \na
              => 10, X, -010, -j

     The representable extrema in the 'i' and 'I' formats correspond to
     Arabic �39,999.  GNU 'troff' uses 'w' and 'z' to represent 5,000
     and 10,000 in Roman numerals, respectively, following the
     convention of AT&T 'troff'--currently, the correct glyphs for Roman
     numerals five thousand ('U+2181') and ten thousand ('U+2182') are
     not used.

     Assigning the format of a read-only register is an error.  Instead,
     copy the read-only register's value to, and assign the format of, a
     writable register.

 -- Escape sequence: \gr
 -- Escape sequence: \g(rg
 -- Escape sequence: \g[reg]
     Interpolate the format of the register REG (one-character name R,
     two-character name RG).  Zeroes represent Arabic formats.  If REG
     is not defined, REG is not created and nothing is interpolated.
     '\g' is interpreted even in copy mode (*note Copy Mode::).

   GNU 'troff' interprets only Arabic numerals.  The Roman numeral or
alphabetic formats cannot be used as operands to arithmetic operators in
expressions (*note Numeric Expressions::).  For instance, it may be
desirable to test the page number independently of its format.

     .af % i \" front matter
     .de header-trap
     .  \" To test the page number, we need it in Arabic.
     .  ds saved-page-number-format \\g%\"
     .  af % 0
     .  nr page-number-in-decimal \\n%
     .  af % \\*[saved-page-number-format]
     .  ie \\n[page-number-in-decimal]=1 .do-first-page-stuff
     .  el \{\
     .    ie o .do-odd-numbered-page-stuff
     .    el   .do-even-numbered-page-stuff
     .  \}
     .  rm saved-page-number-format
     ..
     .wh 0 header-trap

5.8.5 Built-in Registers
------------------------

Predefined registers whose identifiers start with a dot are read-only.
Many are Boolean-valued, interpolating a true or false value testable
with the 'if', 'ie', or 'while' requests.  Some read-only registers are
string-valued, meaning that they interpolate text.

   *Caution:* Built-in registers are subject to removal like others;
once removed, they can be recreated only as normal writable registers
and will not reflect formatter state.

   A register name (without the dot) is often associated with a request
of the same name.  A complete listing of all built-in registers can be
found in *note Register Index::.

   We present here a few built-in registers that are not described
elsewhere in this manual; they have to do with invariant properties of
GNU 'troff', or obtain information about the formatter's command-line
options, processing progress, or the operating environment.

'\n[.A]'
     Approximate output is being formatted (Boolean-valued); see 'groff'
     '-a' option (*note Groff Options::).

'\n[.c]'
'\n[c.]'
     Input line number.  'c.' is a writable synonym, affecting
     subsequent interpolations of both '.c' and 'c.'.

'\n[.F]'
     Name of input file (string-valued).

'\n[.g]'
     Always true in GNU 'troff' (Boolean-valued).  Documents can use
     this to ask the formatter if it claims 'groff' compatibility.

'\n[.P]'
     Output page selection status (Boolean-valued); see 'groff' '-o'
     option (*note Groff Options::).

'\n[.R]'
     Count of available unused registers; always 10,000 in GNU
     'troff'.(1)  (*note Built-in Registers-Footnote-1::)

'\n[.T]'
     Indicator of output device selection (Boolean-valued); see 'groff'
     '-T' option (*note Groff Options::).

'\n[.U]'
     Unsafe mode enablement status (Boolean-valued); see 'groff' '-U'
     option (*note Groff Options::).

'\n[.x]'
     Major version number of the running GNU 'troff' formatter.  For
     example, if the version number is 1.23.0, then '.x' contains '1'.

'\n[.y]'
     Minor version number of the running GNU 'troff' formatter.  For
     example, if the version number is 1.23.0, then '.y' contains '23'.

'\n[.Y]'
     Revision number of the running GNU 'troff' formatter.  For example,
     if the version number is 1.23.0, then '.Y' contains '0'.

'\n[$$]'
     Process identifier (PID) of the GNU 'troff' program in its
     operating environment.

   Date- and time-related registers are set per the local time as
determined by 'localtime(3)' when the formatter launches.  This
initialization can be overridden by 'SOURCE_DATE_EPOCH' and 'TZ'; see
*note Environment::.

'\n[seconds]'
     Count of seconds elapsed in the minute (0-60).

'\n[minutes]'
     Count of minutes elapsed in the hour (0-59).

'\n[hours]'
     Count of hours elapsed since midnight (0-23).

'\n[dw]'
     Day of the week (1-7; 1 is Sunday).

'\n[dy]'
     Day of the month (1-31).

'\n[mo]'
     Month of the year (1-12).

'\n[year]'
     Gregorian year.

'\n[yr]'
     Gregorian year minus 1900.  This register is incorrectly documented
     in the AT&T 'troff' manual as storing the last two digits of the
     current year.  That claim stopped being true in 2000.  Old 'troff'
     input that looks like:

          '\" The year number is a surprise after 1999.
          This document was formatted in 19\n(yr.

     can be corrected to:

          This document was formatted in \n[year].

     or, for portability across many 'roff' programs, to the following.

          .nr y4 1900+\n(yr
          This document was formatted in \n(y4.

   (1) GNU 'troff' dynamically allocates memory for as many registers as
required.

5.9 Manipulating Filling and Adjustment
=======================================

When an output line is pending (see below), a break moves the drawing
position to the beginning of the next text baseline, interrupting
filling.  Various ways of causing breaks were shown in *note Breaking::.
The 'br' request likewise causes a break.  Several other requests imply
breaks: 'bp', 'ce', 'cf', 'fi', 'fl', 'in', 'nf', 'rj', 'sp', 'ti', and
'trf'.  If the no-break control character is used with any of these
requests, GNU 'troff' suppresses the break; instead the requested
operation takes effect at the next break.  ''br' does nothing.

     .ll 55n
     This line is normally filled and adjusted.
     .br
     A line's alignment is decided
     'ce \" Center the next input line (no break).
     when it is output.
     This line returns to normal filling and adjustment.
         => This line is normally filled and adjusted.
         =>    A line's alignment is decided when it is output.
         => This line returns to normal filling and adjustment.

Output line properties like page offset, indentation, adjustment, and
even the location of its text baseline, are not determined until the
line has been broken.  An output line is said to be "pending" if some
input has been collected but an output line corresponding to it has not
yet been written; such an output line is also termed "partially
collected".  If no output line is pending, it is as if a break has
already happened; additional breaks, whether explicit or implicit, have
no effect.  If the vertical drawing position is negative--as it is when
the formatter starts up--a break starts a new page (even if no output
line is pending) unless an end-of-input macro is being interpreted.
*Note End-of-input Traps::.

 -- Request: .br
     Break the line: emit any pending output line without adjustment.

          foo bar
          .br
          baz
          'br
          qux
              => foo bar
              => baz qux

   Sometimes you want to prevent a break within a phrase or between a
quantity and its units.

 -- Escape sequence: \~
     Insert an unbreakable space that is adjustable like an ordinary
     space.  It is discarded from the end of an output line if a break
     is forced.

          Set the output speed to\~1.
          There are 1,024\~bytes in 1\~KiB.
          J.\~F.\~Ossanna wrote the original CSTR\~#54.

   By default, GNU 'troff' fills text and adjusts it to reach the output
line length.  The 'nf' request disables filling; the 'fi' request
re�nables it.

 -- Request: .fi
 -- Register: \n[.u]
     Enable filling of output lines; a pending output line is broken.
     The read-only register '.u' is set to 1.  The filling enablement
     status, sometimes called "fill mode", is associated with the
     environment (*note Environments::).  *Note Line Continuation::, for
     interaction with the '\c' escape sequence.

 -- Request: .nf
     Disable filling of output lines: the output line length (*note Line
     Layout::) is ignored and output lines are broken where the input
     lines are.  A pending output line is broken and adjustment is
     suppressed.  The read-only register '.u' is set to 0.  The filling
     enablement status is associated with the environment (*note
     Environments::).  See *note Line Continuation::, for interaction
     with the '\c' escape sequence.

 -- Request: .ad [mode]
 -- Register: \n[.j]
     Enable output line adjustment in MODE, taking effect when the
     pending (or next) output line is broken.  Adjustment is suppressed
     when filling is.  MODE can have one of the following values.

     'b'
     'n'
          Adjust "normally": if the output line does not consume the
          distance between the indentation and the configured output
          line length, GNU 'troff' stretches adjustable spaces within
          the line until that length is reached.  When the indentation
          is zero, this mode spreads the line to both the left and right
          margins.  This is the GNU 'troff' default.

     'c'
          Center filled text.  Contrast with the 'ce' request, which
          centers text _without_ filling it.

     'l'
          Align text to the left without adjusting it.

     'r'
          Align text to the right without adjusting it.

     MODE can also be a value previously stored in the '.j' register.
     Using 'ad' without an argument is the same as '.ad \n[.j]'; unless
     filling is disabled, GNU 'troff' resumes adjusting lines in the
     same way it did before adjustment was disabled by invocation of the
     'na' request.

     The adjustment mode and enablement status are encoded in the
     read-only register '.j'.  These parameters are associated with the
     environment (*note Environments::).

     The value of '.j' for any adjustment mode is an implementation
     detail and should not be relied upon as a programmer's interface.
     Do not write logic to interpret or perform arithmetic on it.

          .ll 48n
          .de AD
          .  br
          .  ad \\$1
          ..
          .de NA
          .  br
          .  na
          ..
          left
          .AD r
          .nr ad \n(.j
          right
          .AD c
          center
          .NA
          left
          .AD
          center
          .AD \n(ad
          right
              => left
              =>                                            right
              =>                      center
              => left
              =>                      center
              =>                                            right

 -- Request: .na
     Disable output line adjustment.  This produces the same output as
     left-alignment, but the value of the adjustment mode register '.j'
     is altered differently.  The adjustment mode and enablement status
     are associated with the environment (*note Environments::).

 -- Request: .brp
 -- Escape sequence: \p
     Break, adjusting the line per the current adjustment mode.  '\p'
     schedules a break with adjustment at the next word boundary.  The
     escape sequence is itself neither a break nor a space of any kind;
     it can thus be placed in the middle of a word to cause a break at
     the end of that word.

     Breaking with immediate adjustment can produce ugly results since
     GNU 'troff' doesn't have a sophisticated paragraph-building
     algorithm, as TeX has, for example.  Instead, GNU 'troff' fills and
     adjusts a paragraph line by line.

          .ll 4.5i
          This is an uninteresting sentence.
          This is an uninteresting sentence.\p
          This is an uninteresting sentence.

     is formatted as follows.

          This  is  an uninteresting sentence.  This is
          an          uninteresting           sentence.
          This is an uninteresting sentence.

   To clearly present the next couple of requests, we must introduce the
concept of "productive" input lines.  A "productive input line" is one
that directly produces formatted output.  Text lines produce output,(1)
(*note Manipulating Filling and Adjustment-Footnote-1::) as do control
lines containing requests like 'tl' or escape sequences like '\D'.
Macro calls are not _directly_ productive, and thus not counted, but
their interpolated contents can be.  Empty requests, and requests and
escape sequences that define registers or strings or alter the
formatting environment (as with changes to the size, face, height,
slant, or color of the type) are not productive.  We will also preview
the output line continuation escape sequence, '\c', which "connects" two
input lines that would otherwise be counted separately.  (2)  (*note
Manipulating Filling and Adjustment-Footnote-2::)

     .de hello
     Hello, world!
     ..
     .ce \" center output of next productive input line
     .
     .nr junk-reg 1
     .ft I
     Chorus: \c
     .ft
     .hello
     Went the day well?
       =>                  Chorus: Hello, world!
       => Went the day well?

 -- Request: .ce [n]
 -- Register: \n[.ce]
     Break (unless the no-break control character is used), center the
     output of the next N productive input lines with respect to the
     line length and indentation without filling, then break again
     regardless of the invoking control character.  If the argument is
     not positive, centering is disabled.  Omitting the argument implies
     an N of '1'.  The count of lines remaining to be centered is stored
     in the read-only register '.ce' and is associated with the
     environment (*note Environments::).

     While the '.ad c' request also centers text, it fills the text as
     well.

          .de FR
          This is a small text fragment that shows the differences
          between the `.ce' and the `.ad c' requests.
          ..
          .ll 4i
          .ce 1000
          .FR
          .ce 0

          .ad c
          .FR
              => This is a small text fragment that shows
              =>              the differences
              => between the `.ce' and the `.ad c' requests.
              =>
              => This is a small text fragment that shows
              =>  the differences between the `.ce' and
              =>         the `.ad c' requests.

     The previous example illustrates a common idiom of turning
     centering on for a quantity of lines far in excess of what is
     required, and off again after the text to be centered.  This
     technique relieves humans of counting lines for requests that take
     a count of input lines as an argument.

 -- Request: .rj [n]
 -- Register: \n[.rj]
     Break (unless the no-break control character is used), align the
     output of the next N productive input lines to the right margin
     without filling, then break again regardless of the control
     character.  If the argument is not positive, right-alignment is
     disabled.  Omitting the argument implies an N of '1'.  The count of
     lines remaining to be right-aligned is stored in the read-only
     register '.rj' and is associated with the environment (*note
     Environments::).

          .ll 49n
          .rj 3
          At first I hoped that such a technically unsound
          project would collapse but I soon realized it was
          doomed to success. \[em] C. A. R. Hoare
              =>  At first I hoped that such a technically unsound
              => project would collapse but I soon realized it was
              =>              doomed to success. -- C. A. R. Hoare

 -- Request: .ss word-space-size [additional-sentence-space-size]
 -- Register: \n[.ss]
 -- Register: \n[.sss]
     Set the sizes of spaces between words and sentences(3) (*note
     Manipulating Filling and Adjustment-Footnote-3::) in twelfths of
     font's space width (typically one-fourth to one-third em for
     Western scripts).  The default for both parameters is 12.  Negative
     values are erroneous.  The first argument is a minimum; if an
     output line undergoes adjustment, such spaces may increase in
     width.  The optional second argument sets the amount of additional
     space separating sentences on the same output line.  If omitted,
     this amount is set to WORD-SPACE-SIZE.  The request is ignored if
     there are no parameters.

     Additional inter-sentence space is used only if the output line is
     not full when the end of a sentence occurs in the input.  If a
     sentence ends at the end of an input line, then both an inter-word
     space and an inter-sentence space are added to the output; if two
     spaces follow the end of a sentence in the middle of an input line,
     then the second space becomes an inter-sentence space in the
     output.  Additional inter-sentence space is not adjusted, but the
     inter-word space that always precedes it may be.  Further input
     spaces after the second, if present, are adjusted as normal.

     The read-only registers '.ss' and '.sss' hold the minimal
     inter-word space and additional inter-sentence space amounts,
     respectively.  These parameters are part of the environment (*note
     Environments::), and rounded down to the nearest multiple of 12 on
     terminals.

     The 'ss' request can insert discardable horizontal space; that is,
     space that is discarded at a break.  For example, some footnote
     styles collect the notes into a single paragraph with large gaps
     between each note.

          .ll 48n
          1.\~J. Fict. Ch. Soc. 6 (2020), 3\[en]14.
          .ss 12 48 \" applies to next sentence ending
          Reprints no longer available through FCS.
          .ss 12 \" go back to normal
          2.\~Better known for other work.
              => 1.  J.  Fict. Ch. Soc. 6 (2020), 3-14.  Reprints
              => no longer available through FCS.      2.  Better
              => known for other work.

     If _undiscardable_ space is required, use the '\h' escape sequence.

   (1) unless diverted; see *note Diversions::

   (2) *Note Line Continuation::.

   (3) Recall *note Filling:: and *note Sentences:: for the definitions
of word and sentence boundaries, respectively.

5.10 Manipulating Hyphenation
=============================

When filling, GNU 'troff' hyphenates words as needed at user-specified
and automatically determined hyphenation points.  The machine-driven
determination of hyphenation points in words requires algorithms and
data, and is susceptible to conventions and preferences.  Before
tackling such "automatic hyphenation", let us consider how hyphenation
points can be set explicitly.

   Explicitly hyphenated words such as "mother-in-law" are eligible for
breaking after each of their hyphens.  Relatively few words in a
language offer such obvious break points, however, and automatic
detection of syllabic (or phonetic) boundaries for hyphenation is not
perfect,(1) (*note Manipulating Hyphenation-Footnote-1::) particularly
for unusual words found in technical literature.  We can instruct GNU
'troff' how to hyphenate specific words if the need arises.

 -- Request: .hw word ...
     Define each "hyphenation exception" WORD with each hyphen '-' in
     the word indicating a hyphenation point.  For example, the request

          .hw in-sa-lub-rious alpha

     marks potential hyphenation points in "insalubrious", and prevents
     "alpha" from being hyphenated at all.

     Besides the space character, any character whose hyphenation code
     is zero can be used to separate the arguments of 'hw' (see the
     'hcode' request below).  In addition, this request can be used more
     than once.

     Hyphenation points specified with 'hw' are not subject to the
     within-word placement restrictions imposed by the 'hy' request (see
     below).

     Hyphenation exceptions specified with the 'hw' request are
     associated with the hyphenation language (see the 'hla' request
     below) and environment (*note Environments::); invoking the 'hw'
     request in the absence of a hyphenation language is an error.

     The request is ignored if there are no parameters.

   These are known as hyphenation exceptions in the expectation that
most users will avail themselves of automatic hyphenation; these
exceptions override any rules that would normally apply to a word
matching a hyphenation exception defined with 'hw'.

   Situations also arise when only a specific occurrence of a word needs
its hyphenation altered or suppressed, or when a URL or similar string
needs to be breakable in sensible places without hyphenation.

 -- Escape sequence: \%
 -- Escape sequence: \:
     To tell GNU 'troff' how to hyphenate words as they occur in input,
     use the '\%' escape sequence; it is the default "hyphenation
     character".  Each instance within a word indicates to GNU 'troff'
     that the word may be hyphenated at that point, while prefixing a
     word with this escape sequence prevents it from being otherwise
     hyphenated.  This mechanism affects only that occurrence of the
     word; to change the hyphenation of a word for the remainder of
     input processing, use the 'hw' request.

     GNU 'troff' regards the escape sequences '\X' and '\Y' as starting
     a word; that is, the '\%' escape sequence in, say,
     '\X'...'\%foobar' or '\Y'...'\%foobar' no longer prevents
     hyphenation of 'foobar' but inserts a hyphenation point just prior
     to it; most likely this isn't what you want.  *Note Postprocessor
     Access::.

     '\:' inserts a non-printing break point; that is, a word can break
     there, but the soft hyphen glyph (see below) is not written to the
     output if it does.  This escape sequence is an input word boundary,
     so the remainder of the word is subject to hyphenation as normal.

     You can combine '\:' and '\%' to control breaking of a file name or
     URL, or to permit hyphenation only after certain explicit hyphens
     within a word.

          The \%Lethbridge-Stewart-\:\%Sackville-Baggins divorce
          was, in retrospect, inevitable once the contents of
          \%/var/log/\:\%httpd/\:\%access_log on the family web
          server came to light, revealing visitors from Hogwarts.

 -- Request: .hc [char]
     Change the hyphenation character to CHAR.  This character then
     works as the '\%' escape sequence normally does, and thus no longer
     appears in the output.(2)  (*note Manipulating
     Hyphenation-Footnote-2::) Without an argument, 'hc' resets the
     hyphenation character to '\%' (the default).  The hyphenation
     character is associated with the environment (*note
     Environments::).

 -- Request: .shc [c]
     Set the "soft hyphen character", inserted when a word is hyphenated
     automatically or at a hyphenation character, to the ordinary or
     special character C.(3)  (*note Manipulating
     Hyphenation-Footnote-3::) If the argument is omitted, the soft
     hyphen character is set to the default, '\[hy]'.  If no glyph for C
     exists in the font in use at a potential hyphenation point, then
     the line is not broken there.  Neither character definitions
     (specified with the 'char' and similar requests) nor translations
     (specified with the 'tr' request) are applied to C.

   Several requests influence automatic hyphenation.  Because
conventions vary, a variety of hyphenation modes is available to the
'hy' request; these determine whether hyphenation will apply to a word
prior to breaking a line at the end of a page (more or less; see below
for details), and at which positions within that word automatically
determined hyphenation points are permissible.  The places within a word
that are eligible for hyphenation are determined by language-specific
data and lettercase relationships.  Furthermore, hyphenation of a word
might be suppressed due to a limit on consecutive hyphenated lines
('hlm'), a minimum line length threshold ('hym'), or because the line
can instead be adjusted with additional inter-word space ('hys').

 -- Request: .hy [mode]
 -- Register: \n[.hy]
     Set automatic hyphenation mode to MODE, an integer encoding
     conditions for hyphenation; if omitted, '1' is implied.  The
     hyphenation mode is available in the read-only register '.hy'; it
     is associated with the environment (*note Environments::).  The
     default hyphenation mode depends on the localization file loaded
     when GNU 'troff' starts up; see the 'hpf' request below.

     Typesetting practice generally does not avail itself of every
     opportunity for hyphenation, but the details differ by language and
     site mandates.  The hyphenation modes of AT&T 'troff' were
     implemented with English-language publishing practices of the 1970s
     in mind, not a scrupulous enumeration of conceivable parameters.
     GNU 'troff' extends those modes such that finer-grained control is
     possible, favoring compatibility with older implementations over a
     more intuitive arrangement.  The means of hyphenation mode control
     is a set of numbers that can be added up to encode the behavior
     sought.(4)  (*note Manipulating Hyphenation-Footnote-4::) The
     entries in the following table are termed "values"; the sum of the
     desired values is the "mode".

     '0'
          disables hyphenation.

     '1'
          enables hyphenation except after the first and before the last
          character of a word.

     The remaining values "imply" 1; that is, they enable hyphenation
     under the same conditions as '.hy 1', and then apply or lift
     restrictions relative to that basis.

     '2'
          disables hyphenation of the last word on a page,(5) (*note
          Manipulating Hyphenation-Footnote-5::) even for explicitly
          hyphenated words.

     '4'
          disables hyphenation before the last two characters of a word.

     '8'
          disables hyphenation after the first two characters of a word.

     '16'
          enables hyphenation before the last character of a word.

     '32'
          enables hyphenation after the first character of a word.

     Apart from value 2, restrictions imposed by the hyphenation mode
     are _not_ respected for words whose hyphenations have been
     specified with the hyphenation character ('\%' by default) or the
     'hw' request.

     Nonzero values in the previous table are additive.  For example,
     mode 12 causes GNU 'troff' to hyphenate neither the last two nor
     the first two characters of a word.  Some values cannot be used
     together because they contradict; for instance, values 4 and 16,
     and values 8 and 32.  As noted, it is superfluous to add 1 to any
     non-zero even mode.

     The automatic placement of hyphens in words is determined by
     "pattern files", which are derived from TeX and available for
     several languages.  The number of characters at the beginning of a
     word after which the first hyphenation point should be inserted is
     determined by the patterns themselves; it can't be reduced further
     without introducing additional, invalid hyphenation points
     (unfortunately, this information is not part of a pattern file--you
     have to know it in advance).  The same is true for the number of
     characters at the end of a word before the last hyphenation point
     should be inserted.  For example, you can supply the following
     input to 'echo $(nroff)'.

          .ll 1
          .hy 48
          splitting

     You will get

          s- plit- t- in- g

     instead of the correct 'split- ting'.  English patterns as
     distributed with GNU 'troff' need two characters at the beginning
     and three characters at the end; this means that value 4 of 'hy' is
     mandatory.  Value 8 is possible as an additional restriction, but
     values 16 and 32 should be avoided, as should mode 1.  Modes 4
     and 6 are typical.

     A table of left and right minimum character counts for hyphenation
     as needed by the patterns distributed with GNU 'troff' follows; see
     the 'groff_tmac(5)' man page for more information on GNU 'troff''s
     language macro files.

     language             pattern name   left min   right min
     -----------------------------------------------------------
     Czech                cs             2          2
     English              en             2          3
     French               fr             2          3
     German traditional   det            2          2
     German reformed      den            2          2
     Italian              it             2          2
     Swedish              sv             1          2

     Hyphenation exceptions within pattern files (i.e., the words within
     a TeX '\hyphenation' group) obey the hyphenation restrictions given
     by 'hy'.

 -- Request: .nh
     Disable automatic hyphenation; i.e., set the hyphenation mode to 0
     (see above).  The hyphenation mode of the last call to 'hy' is not
     remembered.

 -- Request: .hpf pattern-file
 -- Request: .hpfa pattern-file
 -- Request: .hpfcode a b [c d] ...
     Read hyphenation patterns from PATTERN-FILE, which is sought in the
     same way that macro files are with the 'mso' request or the
     '-mNAME' command-line option to 'groff'.  The PATTERN-FILE should
     have the same format as (simple) TeX pattern files.  More
     specifically, the following scanning rules are implemented.

        * A percent sign starts a comment (up to the end of the line)
          even if preceded by a backslash.

        * "Digraphs" like '\$' are not supported.

        * '^^XX' (where each X is 0-9 or a-f) and '^^C' (character C in
          the code point range 0-127 decimal) are recognized; other uses
          of '^' cause an error.

        * No macro expansion is performed.

        * 'hpf' checks for the expression '\patterns{...}' (possibly
          with whitespace before or after the braces).  Everything
          between the braces is taken as hyphenation patterns.
          Consequently, '{' and '}' are not allowed in patterns.

        * Similarly, '\hyphenation{...}' gives a list of hyphenation
          exceptions.

        * '\endinput' is recognized also.

        * For backward compatibility, if '\patterns' is missing, the
          whole file is treated as a list of hyphenation patterns
          (except that the '%' character is recognized as the start of a
          comment).

     The 'hpfa' request appends a file of patterns to the current list.

     The 'hpfcode' request defines mapping values for character codes in
     pattern files.  It is an older mechanism no longer used by GNU
     'troff''s own macro files; for its successor, see 'hcode' below.
     'hpf' or 'hpfa' apply the mapping after reading the patterns but
     before replacing or appending to the active list of patterns.  Its
     arguments are pairs of character codes--integers from 0 to 255.
     The request maps character code A to code B, code C to code D, and
     so on.  Character codes that would otherwise be invalid in GNU
     'troff' can be used.  By default, every code maps to itself except
     those for letters 'A' to 'Z', which map to those for 'a' to 'z'.

     The set of hyphenation patterns is associated with the language set
     by the 'hla' request (see below).  The 'hpf' request is usually
     invoked by a localization file loaded by the 'troffrc' file.(6)
     (*note Manipulating Hyphenation-Footnote-6::)

     A second call to 'hpf' (for the same language) replaces the
     hyphenation patterns with the new ones.  Invoking 'hpf' or 'hpfa'
     causes an error if there is no hyphenation language.  If no 'hpf'
     request is specified (either in the document, in a file loaded at
     startup, or in a macro package), GNU 'troff' won't automatically
     hyphenate at all.

 -- Request: .hcode c1 code1 [c2 code2] ...
     Set the hyphenation code of character C1 to CODE1, that of C2 to
     CODE2, and so on.  A hyphenation code must be an ordinary character
     (not a special character escape sequence) other than a digit or a
     space.  The request is ignored if given no arguments.

     For hyphenation to work, hyphenation codes must be set up.  At
     startup, GNU 'troff' assigns hyphenation codes to the letters
     'a'-'z' (mapped to themselves), to the letters 'A'-'Z' (mapped to
     'a'-'z'), and zero to all other characters.  Normally, hyphenation
     patterns contain only lowercase letters which should be applied
     regardless of case.  In other words, they assume that the words
     'FOO' and 'Foo' should be hyphenated exactly as 'foo' is.  The
     'hcode' request extends this principle to letters outside the
     Unicode basic Latin alphabet; without it, words containing such
     letters won't be hyphenated properly even if the corresponding
     hyphenation patterns contain them.

     For example, the following 'hcode' requests are necessary to assign
     hyphenation codes to the letters '�������', needed for German.

          .hcode � �  � �
          .hcode � �  � �
          .hcode � �  � �
          .hcode � �

     Without these assignments, GNU 'troff' treats the German word
     'Kinderg�rten' (the plural form of 'kindergarten') as two words
     'kinderg' and 'rten' because the hyphenation code of the umlaut a
     is zero by default, just like a space.  There is a German
     hyphenation pattern that covers 'kinder', so GNU 'troff' finds the
     hyphenation 'kin-der'.  The other two hyphenation points
     ('kin-der-g�r-ten') are missed.

 -- Request: .hla lang
 -- Register: \n[.hla]
     Set the hyphenation language to LANG.  Hyphenation exceptions
     specified with the 'hw' request and hyphenation patterns and
     exceptions specified with the 'hpf' and 'hpfa' requests are
     associated with the hyphenation language.  The 'hla' request is
     usually invoked by a localization file, which is turn loaded by the
     'troffrc' or 'troffrc-end' file; see the 'hpf' request above.

     The hyphenation language is available in the read-only
     string-valued register '.hla'; it is associated with the
     environment (*note Environments::).

 -- Request: .hlm [n]
 -- Register: \n[.hlm]
 -- Register: \n[.hlc]
     Set the maximum quantity of consecutive hyphenated lines to N.  If
     N is negative, there is no maximum.  If omitted, N is -1.  This
     value is associated with the environment (*note Environments::).
     Only lines output from a given environment count toward the maximum
     associated with that environment.  Hyphens resulting from '\%' are
     counted; explicit hyphens are not.

     The '.hlm' read-only register stores this maximum.  The count of
     immediately preceding consecutive hyphenated lines is available in
     the read-only register '.hlc'.

 -- Request: .hym [length]
 -- Register: \n[.hym]
     Set the (right) hyphenation margin to LENGTH.  If the adjustment
     mode is not 'b' or 'n', the line is not hyphenated if it is shorter
     than LENGTH.  Without an argument, the hyphenation margin is reset
     to its default value, 0.  The default scaling unit is 'm'.  The
     hyphenation margin is associated with the environment (*note
     Environments::).

     A negative argument resets the hyphenation margin to zero, emitting
     a warning in category 'range'.

     The hyphenation margin is available in the '.hym' read-only
     register.

 -- Request: .hys [hyphenation-space]
 -- Register: \n[.hys]
     Suppress hyphenation of the line in adjustment modes 'b' or 'n' if
     it can be justified by adding no more than HYPHENATION-SPACE extra
     space to each inter-word space.  Without an argument, the
     hyphenation space adjustment threshold is set to its default value,
     0.  The default scaling unit is 'm'.  The hyphenation space
     adjustment threshold is associated with the environment (*note
     Environments::).

     A negative argument resets the hyphenation space adjustment
     threshold to zero, emitting a warning in category 'range'.

     The hyphenation space adjustment threshold is available in the
     '.hys' read-only register.

   (1) Whether a perfect algorithm for this application is even possible
is an unsolved problem in computer science:
<https://tug.org/docs/liang/liang-thesis.pdf>.

   (2) '\%' itself stops marking hyphenation points but still produces
no output glyph.

   (3) "Soft" because it appears in output only where a hyphenation
break is performed; a "hard" hyphen, as in "long-term", always appears.

   (4) The mode is a vector of Booleans encoded as an integer.  To a
programmer, this fact is easily deduced from the exclusive use of powers
of two for the configuration parameters; they are computationally easy
to "mask off" and compare to zero.  To almost everyone else, the
arrangement seems recondite and unfriendly.

   (5) Hyphenation is prevented if the next page location trap is closer
to the vertical drawing position than the next text baseline would be.
*Note Page Location Traps::.

   (6) For more on localization, see the 'groff_tmac(5)' man page.

5.11 Manipulating Spacing
=========================

A break causes the formatter to update the vertical drawing position at
which the new text baseline is aligned.  You can alter this location.

 -- Request: .sp [distance]
     Break and move the next text baseline down by DISTANCE, or until
     springing a page location trap.(1)  (*note Manipulating
     Spacing-Footnote-1::) If invoked with the no-break control
     character, 'sp' moves the pending output line's text baseline by
     DISTANCE.  A negative DISTANCE will not reduce the position of the
     text baseline below zero.  Inside a diversion, any DISTANCE
     argument is ignored.  The default scaling unit is 'v'.  If DISTANCE
     is not specified, '1v' is assumed.

          .pl 5v \" Set page length to 5 vees.
          .de xx
          \-\-\-
          .  br
          ..
          .wh 0 xx \" Set a trap at the top of the page.
          foo on page \n%
          .sp 2v
          bar on page \n%
          .sp 50v \" This will cause a page break.
          baz on page \n%
          .pl \n(nlu \" Truncate page to current position.
              => ---
              => foo on page 1
              =>
              =>
              => bar on page 1
              => ---
              => baz on page 2

     You might use the following macros to set the baseline of the next
     output text at a given distance from the top or the bottom of the
     page.  We subtract one line height ('\n[.v]') because the '|'
     operator moves to one vee below the page top (recall *note Numeric
     Expressions::).

          .de y-from-top-down
          .  sp |\\$1-\\n[.v]u
          ..
          .
          .de y-from-bot-up
          .  sp |\\n[.p]u-\\$1-\\n[.v]u
          ..

     A call to '.y-from-bot-up 10c' means that the next text baseline
     will be 10 cm from the bottom edge of the paper.

 -- Request: .ls [count]
 -- Register: \n[.L]
     Set the line spacing; add COUNT-1 blank lines after each line of
     text.  With no argument, GNU 'troff' uses the previous value before
     the last 'ls' call.  The default is '1'.

     The read-only register '.L' contains the current line spacing; it
     is associated with the environment (*note Environments::).

   The 'ls' request is a coarse mechanism.  *Note Changing the Type
Size::, for the requests 'vs' and 'pvs' as alternatives to 'ls'.

 -- Escape sequence: \x'spacing'
 -- Register: \n[.a]
     Sometimes, an output line requires additional vertical spacing, for
     instance to allow room for a tall construct like an inline equation
     with exponents or subscripts (particularly if they are iterated).
     The '\x' escape sequence takes a delimited measurement (like
     '\x'3p'') to increase the vertical spacing of the pending output
     line.  The default scaling unit is 'v'.  If the measurement is
     positive, extra vertical space is inserted below the current line;
     a negative measurement adds space above.  If '\x' is applied to the
     pending output line multiple times, the maxima of the positive and
     negative adjustments are separately applied.  The delimiter need
     not be a neutral apostrophe; see *note Delimiters::.

     The '.a' read-only register contains the extra vertical spacing
     _after_ the text baseline of the most recently emitted output line.
     (In other words, it is the largest positive argument to '\x'
     encountered on that line.)  This quantity is exposed via a register
     because if an output line requires this "extra post-vertical line
     spacing", and the subsequent output line requires "extra
     pre-vertical line spacing" (a negative argument to '\x'), then
     applying both can lead to excessive spacing between the output
     lines.  Text that is piling high on line N might not require (as
     much) extra pre-vertical line spacing if line N-1 carries extra
     post-vertical line spacing.

     Use of '\x' can be necessary in combination with the
     bracket-building escape sequence '\b',(2) (*note Manipulating
     Spacing-Footnote-2::) as the following example shows.

          .nf
          This is a test of \[rs]b (1).
          This is a test of \[rs]b (2).
          This is a test of \b'xyz'\x'-1m'\x'1m' (3).
          This is a test of \[rs]b (4).
          This is a test of \[rs]b (5).
              => This is a test of \b (1).
              => This is a test of \b (2).
              =>                   x
              => This is a test of y (3).
              =>                   z
              => This is a test of \b (4).
              => This is a test of \b (5).

Without '\x', the backslashes on the lines marked '(2)' and '(4)' would
be overprinted.

 -- Request: .ns
 -- Request: .rs
 -- Register: \n[.ns]
     Enable "no-space mode".  Vertical spacing, whether by 'sp' requests
     or blank input lines, is disabled.  The 'bp' request to advance to
     the next page is also disabled, unless it is accompanied by a page
     number (*note Page Control::).  No-space mode ends automatically
     when text(3) (*note Manipulating Spacing-Footnote-3::) is formatted
     for output (4) (*note Manipulating Spacing-Footnote-4::) or the
     'rs' request is invoked, which ends no-space mode.  The read-only
     register '.ns' interpolates a Boolean value indicating the
     enablement of no-space mode.

     A paragraphing macro might ordinarily insert vertical space to
     separate paragraphs.  A section heading macro could invoke 'ns' to
     suppress this spacing for the first paragraph in a section.

   (1) *Note Page Location Traps::.

   (2) *Note Drawing Geometric Objects::.

   (3) or geometric objects; see *note Drawing Geometric Objects::

   (4) to the top-level diversion; see *note Diversions::

5.12 Tabs and Fields
====================

A tab character (ISO code point 9, EBCDIC code point 5) causes a
horizontal movement to the next tab stop, if any.

 -- Escape sequence: \t
     Interpolate a tab in copy mode; see *note Copy Mode::.

 -- Request: .ta [[n1 n2 ... nn ]T r1 r2 ... rn]
 -- Register: \n[.tabs]
     Change tab stop positions.  This request takes a series of tab
     specifiers as arguments (optionally divided into two groups with
     the letter 'T') that indicate where each tab stop is to be,
     overriding any previous settings.  The default scaling unit is 'm'.
     Invoking 'ta' without an argument removes all tab stops.  GNU
     'troff''s startup value is 'T 0.5i'.

     Tab stops can be specified absolutely--as distances from the left
     margin.  The following example sets six tab stops, one every inch.

          .ta 1i 2i 3i 4i 5i 6i

     Tab stops can also be specified using a leading '+', which means
     that the specified tab stop is set relative to the previous tab
     stop.  For example, the following is equivalent to the previous
     example.

          .ta 1i +1i +1i +1i +1i +1i

     GNU 'troff' supports an extended syntax to specify repeating tab
     stops.  These stops appear after a 'T' argument.  Their values are
     always taken as distances relative to the previous tab stop.  This
     is the idiomatic way to specify tab stops at equal intervals in
     'groff'.  The following is, yet again, the same as the previous
     examples.  It does more, in fact, since it defines an infinite
     number of tab stops at one-inch intervals.

          .ta T 1i

     Now we are ready to interpret the full syntax given above.  The
     'ta' request sets tabs at positions N1, N2, ..., NN, then at NN+R1,
     NN+R2, ..., NN+RN, then at NN+RN+R1, NN+RN+R2, ..., NN+RN+RN, and
     so on.

     For example, '4c +6c T 3c 5c 2c' is equivalent to '4c 10c 13c 18c
     20c 23c 28c 30c ...'.

     Text written to a tab column (i.e., between two tab stops, or
     between a tab stop and an output line boundary) may be aligned to
     the right or left, or centered in the column.  This alignment is
     determined by appending 'R', 'L', or 'C' to the tab specifier.  The
     default is 'L'.

          .ta 1i 2iC 3iR

     The beginning of an output line is not a tab stop; the text that
     begins an output line is placed according to the configured
     alignment and indentation; see *note Manipulating Filling and
     Adjustment:: and *note Line Layout::.

     A tab stop is converted into a non-breakable horizontal movement
     that cannot be adjusted.

          .ll 2i
          .ds foo a\tb\tc
          .ta T 1i
          \*[foo]
              error-> warning: cannot break line
              => a         b         c

     The above creates a single output line that is a bit longer than
     two inches (we use a string to show exactly where the tab stops
     are).  Now consider the following.

          .ll 2i
          .ds bar a\tb c\td
          .ta T 1i
          \*[bar]
              error-> warning: cannot adjust line
              => a         b
              => c       d

     GNU 'troff' first converts the line's tab stops into unbreakable
     horizontal movements, then breaks after 'b'.  This usually isn't
     what you want.

     Superfluous tab characters--those that do not correspond to a tab
     stop--are ignored except for the first, which delimits the
     characters belonging to the last tab stop for right-alignment or
     centering.

          .ds Z   foo\tbar\tbaz
          .ds ZZ  foo\tbar\tbazqux
          .ds ZZZ foo\tbar\tbaz\tqux
          .ta 2i 4iR
          \*[Z]
          .br
          \*[ZZ]
          .br
          \*[ZZZ]
          .br
              => foo                 bar              baz
              => foo                 bar           bazqux
              => foo                 bar              bazqux

     The first line right-aligns "baz" within the second tab stop.  The
     second line right-aligns "bazqux" within it.  The third line
     right-aligns only "baz" because of the additional tab character,
     which marks the end of the text occupying the last tab stop
     defined.

     Tab stops are associated with the environment (*note
     Environments::).

     The read-only register '.tabs' contains a string representation of
     the current tab settings suitable for use as an argument to the
     'ta' request.(1)  (*note Tabs and Fields-Footnote-1::)

          .ds tab-string \n[.tabs]
          \*[tab-string]
              => T120u

 -- Request: .tc [c]
     Set the tab repetition character to the ordinary or special
     character C; normally, no glyph is written when moving to a tab
     stop (and some output devices may output space characters to
     achieve this motion).  A "tab repetition character" causes the
     formatter to write as many instances of C as are necessary to
     occupy the interval from the horizontal drawing position to the
     next tab stop.  With no argument, GNU 'troff' reverts to the
     default behavior.  The tab repetition character is associated with
     the environment (*note Environments::).  Only a single character of
     C is recognized; any excess is ignored.

 -- Request: .linetabs n
 -- Register: \n[.linetabs]
     If N is missing or non-zero, activate "line-tabs"; deactivate it
     otherwise (the default).  Active line-tabs cause GNU 'troff' to
     compute tab distances relative to the start of the output line
     instead of the input line.

          .de Tabs
          .  ds x a\t\c
          .  ds y b\t\c
          .  ds z c
          .  ta 1i 3i
          \\*x
          \\*y
          \\*z
          ..
          .Tabs
          .br
          .linetabs
          .Tabs
              => a         b         c
              => a         b                   c

     Line-tabs activation is associated with the environment (*note
     Environments::).  The read-only register '.linetabs' interpolates 1
     if line-tabs are active, and 0 otherwise.

   (1) Plan 9 'troff' uses the register '.S' for this purpose.

5.12.1 Leaders
--------------

Sometimes it is desirable to fill a tab stop with a given glyph, but
also use tab stops normally on the same output line.  An example is a
table of contents entry that uses dots to bridge the entry name with its
page number, which is itself aligned between tab stops.  The 'roff'
language provides "leaders" for this purpose.(1)  (*note
Leaders-Footnote-1::)

   A leader character (ISO and EBCDIC code point 1, also known as SOH or
"start of heading"), behaves similarly to a tab character: it moves to
the next tab stop.  The difference is that for this movement, the
default fill character is a period '.'.

 -- Escape sequence: \a
     Interpolate a leader in copy mode; see *note Copy Mode::.

 -- Request: .lc [c]
     Set the leader repetition character to the ordinary or special
     character C.  Recall *note Tabs and Leaders::: when encountering a
     leader character in the input, the formatter writes as many dots
     '.' as are necessary until reaching the next tab stop; this is the
     "leader definition character".  Omitting C unsets the leader
     character.  With no argument, GNU 'troff' treats leaders the same
     as tabs.  The leader repetition character is associated with the
     environment (*note Environments::).  Only a single C is recognized;
     any excess is ignored.

   A table of contents, for example, may define tab stops after a
section number, a title, and a gap to be filled with leader dots.  The
page number follows the leader, after a right-aligned final tab stop
wide enough to house the largest page number occurring in the document.

     .ds entry1 19.\tThe Prophet\a\t98
     .ds entry2 20.\tAll Astir\a\t101
     .ta .5i 4.5i +.5iR
     .nf
     \*[entry1]
     \*[entry2]
         => 19.  The Prophet.............................   98
         => 20.  All Astir...............................  101

   (1) This is pronounced to rhyme with "feeder", and refers to how the
glyphs "lead" the eye across the page to the corresponding page number
or other datum.

5.12.2 Fields
-------------

"Fields" are a more general way of laying out tabular data.  A field is
defined as the data between a pair of "delimiting characters".  It
contains substrings that are separated by "padding characters".  The
width of a field is the distance on the _input_ line from the position
where the field starts to the next tab stop.  A padding character
inserts an adjustable space similar to TeX's '\hss' command (thus it can
even be negative) to make the sum of all substring lengths plus the
adjustable space equal to the field width.  If more than one padding
character is inserted, the available space is evenly distributed among
them.

 -- Request: .fc [delim-char [padding-char]]
     Define a delimiting and a padding character for fields.  If the
     latter is missing, the padding character defaults to a space
     character.  If there is no argument at all, the field mechanism is
     disabled (which is the default).  In contrast to, e.g., the tab
     repetition character, delimiting and padding characters are _not_
     associated with the environment (*note Environments::).

          .fc # ^
          .ta T 3i
          #foo^bar^smurf#
          .br
          #foo^^bar^smurf#
              => foo         bar          smurf
              => foo            bar       smurf

5.13 Character Translations
===========================

A "translation" is a mapping of an input character to an output glyph.
The mapping occurs at output time, i.e., the input character gets
assigned the metric information of the mapped output character right
before input tokens are converted to nodes (*note Gtroff Internals::,
for more on this process).

 -- Request: .tr abcd...
 -- Request: .trin abcd...
     Translate character A to glyph B, character C to glyph D, and so
     on.  If there is an odd number of characters in the argument, the
     last one is translated to a fixed-width space (the same one
     obtained by the '\<SP>' escape sequence).

     The 'trin' request is identical to 'tr', but when you unformat a
     diversion with 'asciify' it ignores the translation.  *Note
     Diversions::, for details about the 'asciify' request.

     Some notes:

        * Special characters ('\(XX', '\[XXX]', '\C'XXX'', '\'', '\`',
          '\-', '\_'), glyphs defined with the 'char' request, and
          numbered glyphs ('\N'XXX'') can be translated also.

        * The '\e' escape can be translated also.

        * Characters can be mapped onto the '\%' and '\~' escape
          sequences (but '\%' and '\~' can't be mapped onto another
          glyph).

        * The following characters can't be translated: space (with one
          exception, see below), backspace, newline, leader (and '\a'),
          tab (and '\t').

        * Translations are not considered for finding the soft hyphen
          character set with the 'shc' request.

        * The pair 'C\&' (an arbitrary character C followed by the dummy
          character) maps this character to "nothing".

               .tr a\&
               foo bar
                   => foo br

          Even the space character can be mapped to the dummy character.

               .tr aa \&
               foo bar
                   => foobar

          As shown in the example, the space character can't be the
          first character/glyph pair as an argument of 'tr'.
          Additionally, it is not possible to map the space character to
          any other glyph; requests like '.tr aa x' undo '.tr aa \&'
          instead.

          If justification is active, lines are justified in spite of
          the 'empty' space character (but there is no minimal distance,
          i.e., the space character, between words).

        * After an output glyph has been constructed (this happens at
          the moment immediately before the glyph is appended to an
          output glyph list, either by direct output, in a macro,
          diversion, or string), it is no longer affected by 'tr'.

        * Translating character to glyphs where one of them or both are
          undefined is possible also; 'tr' does not check whether the
          elements of its argument exist.

          *Note Gtroff Internals::.

        * Without an argument, the 'tr' request is ignored.

 -- Request: .trnt abcd...
     'trnt' is the same as the 'tr' request except that the translations
     do not apply to text that is transparently throughput into a
     diversion with '\!'.  *Note Diversions::.

     For example,

          .tr ab
          .di x
          \!.tm a
          .di
          .x

     prints 'b' to the standard error stream; if 'trnt' is used instead
     of 'tr' it prints 'a'.

5.14 'troff' and 'nroff' Modes
==============================

Historically, 'nroff' and 'troff' were two separate programs; the former
for terminal output, the latter for typesetters.  GNU 'troff' merges
both functions into one executable(1) (*note troff and nroff
Modes-Footnote-1::) that sends its output to a device driver ('grotty'
for terminal devices, 'grops' for PostScript, and so on) which
interprets this intermediate output format.  When discussing AT&T
'troff', it makes sense to talk about "'nroff' mode" and "'troff' mode"
since the differences are hard-coded.  GNU 'troff' takes information
from device and font description files without handling requests
specially if a terminal output device is used, so such a strong
distinction is unnecessary.

   Usually, a macro package can be used with all output devices.
Nevertheless, it is sometimes necessary to make a distinction between
terminal and non-terminal devices: GNU 'troff' provides two built-in
conditions 'n' and 't' for the 'if', 'ie', and 'while' requests to
decide whether GNU 'troff' shall behave like 'nroff' or like 'troff'.

 -- Request: .troff
     Make the 't' built-in condition true (and the 'n' built-in
     condition false) for 'if', 'ie', and 'while' conditional requests.
     This is the default if GNU 'troff' (_not_ 'groff') is started with
     the '-R' switch to avoid loading of the startup files 'troffrc' and
     'troffrc-end'.  Without '-R', GNU 'troff' stays in 'troff' mode if
     the output device is not a terminal (e.g., 'ps').

 -- Request: .nroff
     Make the 'n' built-in condition true (and the 't' built-in
     condition false) for 'if', 'ie', and 'while' conditional requests.
     This is the default if GNU 'troff' uses a terminal output device;
     the code for switching to 'nroff' mode is in the file 'tty.tmac',
     which is loaded by the startup file 'troffrc'.

   *Note Conditionals and Loops::, for more details on built-in
conditions.

   (1) A GNU 'nroff' program is available for convenience; it calls GNU
'troff' to perform the formatting.

5.15 Line Layout
================

The following drawing shows the dimensions that 'gtroff' uses for
placing a line of output onto the page.  They are labeled with the
request that manipulates each dimension.

          -->| in |<--
             |<-----------ll------------>|
        +----+----+----------------------+----+
        |    :    :                      :    |
        +----+----+----------------------+----+
     -->| po |<--
        |<--------paper width---------------->|

These dimensions are:

'po'
     "Page offset"--this is the leftmost position of text on the final
     output, defining the "left margin".

'in'
     "Indentation"--this is the distance from the left margin where text
     is printed.

'll'
     "Line length"--this is the distance from the left margin to right
     margin.

   The right margin is not explicitly configured; the combination of
page offset and line length provides the information necessary to derive
it.

   A simple demonstration:

     .ll 3i
     This is text without indentation.
     The line length has been set to 3\~inches.
     .in +.5i
     .ll -.5i
     Now the left and right margins are both increased.
     .in
     .ll
     Calling .in and .ll without parameters restores
     the previous values.

         => This  is text without indenta-
         => tion.   The  line  length  has
         => been set to 3 inches.
         =>      Now   the  left  and
         =>      right  margins   are
         =>      both increased.
         => Calling  .in  and  .ll without
         => parameters restores the previ-
         => ous values.

 -- Request: .po [offset]
 -- Request: .po +offset
 -- Request: .po -offset
 -- Register: \n[.o]
     Set page offset to OFFSET (or increment or decrement its current
     value by OFFSET).  If invoked without an argument, the page offset
     is restored to the value before the previous 'po' request.  This
     request does not cause a break; the page offset in effect when an
     output line is broken prevails (*note Manipulating Filling and
     Adjustment::).  The initial value is 1i and the default scaling
     unit is 'm'.  On terminal devices, the page offset is set to zero
     by a driver-specific macro file, 'tty.tmac'.  The current page
     offset can be found in the read-only register '.o'.  This request
     is incorrectly documented in the AT&T 'troff' manual as using a
     default scaling unit of 'v'.

          .po 3i
          \n[.o]
              => 720
          .po -1i
          \n[.o]
              => 480
          .po
          \n[.o]
              => 720

 -- Request: .in [indent]
 -- Request: .in +indent
 -- Request: .in -indent
 -- Register: \n[.i]
     Set indentation to INDENT (or increment or decrement the current
     value by INDENT).  This request causes a break.  Initially, there
     is no indentation.

     If 'in' is called without an argument, the indentation is reset to
     the previous value before the last call to 'in'.  The default
     scaling unit is 'm'.

     If a negative indentation value is specified (which is not
     allowed), 'gtroff' emits a warning in category 'range' and sets the
     indentation to zero.

     The effect of 'in' is delayed until a partially collected line (if
     it exists) is output.  A temporary indentation value is reset to
     zero also.

     The current indentation (as set by 'in') can be found in the
     read-only register '.i'.  The indentation is associated with the
     environment (*note Environments::).

 -- Request: .ti offset
 -- Request: .ti +offset
 -- Request: .ti -offset
 -- Register: \n[.in]
     Temporarily indent the next output line by OFFSET.  If an increment
     or decrement value is specified, adjust the temporary indentation
     relative to the value set by the 'in' request.

     This request causes a break; its value is associated with the
     environment (*note Environments::).  The default scaling unit is
     'm'.  A call of 'ti' without an argument is ignored.

     If the total indentation value is negative (which is not allowed),
     'gtroff' emits a warning in category 'range' and sets the temporary
     indentation to zero.  'Total indentation' is either OFFSET if
     specified as an absolute value, or the temporary plus normal
     indentation, if OFFSET is given as a relative value.

     The effect of 'ti' is delayed until a partially collected line (if
     it exists) is output.

     The read-only register '.in' is the indentation that applies to the
     current output line.

     The difference between '.i' and '.in' is that the latter takes into
     account whether a partially collected line still uses the old
     indentation value or a temporary indentation value is active.

 -- Request: .ll [length]
 -- Request: .ll +length
 -- Request: .ll -length
 -- Register: \n[.l]
 -- Register: \n[.ll]
     Set the line length to LENGTH (or increment or decrement the
     current value by LENGTH).  Initially, the line length is set to
     6.5i.  The effect of 'll' is delayed until a partially collected
     line (if it exists) is output.  The default scaling unit is 'm'.

     If 'll' is called without an argument, the line length is reset to
     the previous value before the last call to 'll'.  If a negative
     line length is specified (which is not allowed), 'gtroff' emits a
     warning in category 'range' and sets the line length to zero.  The
     line length is associated with the environment (*note
     Environments::).

     The current line length (as set by 'll') can be found in the
     read-only register '.l'.  The read-only register '.ll' is the line
     length that applies to the current output line.

     Similar to '.i' and '.in', the difference between '.l' and '.ll' is
     that the latter takes into account whether a partially collected
     line still uses the old line length value.

5.16 Line Continuation
======================

When filling is enabled, input and output line breaks generally do not
correspond.  The 'roff' language therefore distinguishes input and
output line continuation.

 -- Escape sequence: \<RET>
     '\<RET>' (a backslash immediately followed by a newline) suppresses
     the effects of that newline in the input.  The next input line thus
     retains the classification of its predecessor as a control or text
     line.  '\<RET>' is useful for managing line lengths in the input
     during document maintenance; you can break an input line in the
     middle of a request invocation, macro call, or escape sequence.
     Input line continuation is invisible to the formatter, with two
     exceptions: the '|' operator recognizes the new input line (*note
     Numeric Expressions::), and the input line counter register '.c' is
     incremented.

          .ll 50n
          .de I
          .  ft I
          .  nop \\$*
          .  ft
          ..
          Our film class watched
          .I The Effect of Gamma Rays on Man-in-the-Moon
          Marigolds. \" whoops, the input line wrapped
          .br
          .I My own opus begins on line \n[.c] \
          and ends on line \n[.c].
              => Our film class watched The Effect of Gamma Rays on
              => Man-in-the-Moon Marigolds.
              => My own opus begins on line 11 and ends on line 12.

 -- Escape sequence: \c
 -- Register: \n[.int]
     '\c' continues an output line.  Nothing after it on the input line
     is formatted.  In contrast to '\<RET>', a line after '\c' remains a
     new input line, so a control character is recognized at its
     beginning.  The visual results depend on whether filling is
     enabled; see *note Manipulating Filling and Adjustment::.

        * If filling is enabled, a word interrupted with '\c' is
          continued with the text on the next input text line, without
          an intervening space.

               This is a te\c
               st.
                   => This is a test.

        * If filling is disabled, the next input text line after '\c' is
          handled as a continuation of the same input text line.

               .nf
               This is a \c
               test.
                   => This is a test.

     An intervening control line that causes a break overrides '\c',
     flushing out the pending output line in the usual way.

     The '.int' register contains a positive value if the last output
     line was continued with '\c'; this datum is associated with the
     environment (*note Environments::).(1)  (*note Line
     Continuation-Footnote-1::)

   (1) Historically, the '\c' escape sequence has proven challenging to
characterize.  Some sources say it "connects the next input text" (to
the input line on which it appears); others describe it as
"interrupting" text, on the grounds that a text line is interrupted
without breaking, perhaps to inject a request invocation or macro call.

5.17 Page Layout
================

The formatter permits configuration of the page length and page number.

 -- Request: .pl [length]
 -- Request: .pl +length
 -- Request: .pl -length
 -- Register: \n[.p]
     Change (increase or decrease) the page length per the numeric
     expression LENGTH.  The default scaling unit is 'v'.  A negative
     LENGTH is valid, but an uncommon application: it prevents page
     location traps from being sprung,(1) (*note Page
     Layout-Footnote-1::) and each output line is placed on a new page.
     If LENGTH is invalid, GNU 'troff' emits a warning in category
     'number'.  If LENGTH is absent or invalid, '11i' is assumed.

     The read-only register '.p' interpolates the current page length.

 -- Request: .pn num
 -- Request: .pn +num
 -- Request: .pn -num
 -- Register: \n[.pn]
     Change (increase or decrease) the page number of the _next_ page
     per the numeric expression NUM.  If NUM is invalid, GNU 'troff'
     emits a warning in category 'number' and ignores the request.
     Without an argument, 'pn' is ignored.

     The read-only register '.pn' interpolates NUM if set by 'pn' on the
     current page, or the current page number plus 1.

   The formatter offers special support for typesetting headers and
footers, collectively termed "titles".  Titles have an independent line
length, and their placement on the page is not restricted.

 -- Request: .tl 'left'center'right'
     Format an output line as a title consisting of LEFT, CENTER, and
     RIGHT, each aligned accordingly.  The delimiter need not be a
     neutral apostrophe: 'tl' accepts the same delimiters as most escape
     sequences; see *note Delimiters::.  If not used as the delimiter,
     any "page number character" character is replaced with the current
     page number; the default is '%'; see the the 'pc' request below.
     Without an argument, 'tl' is ignored.  'tl' writes the title line
     immediately, ignoring any partially collected line.

     It is not an error to omit delimiters after the first.  For
     example, '.tl /Thesis' is interpreted as '.tl /Thesis///': it sets
     a title line comprising only the left-aligned word 'Thesis'.

 -- Request: .lt [length]
 -- Request: .lt +length
 -- Request: .lt -length
 -- Register: \n[.lt]
     Change (increase or decrease) the line length used by titles per
     the numeric expression LENGTH.  The default scaling unit is 'm'.
     If LENGTH is negative, GNU emits a warning in category 'range' and
     treats LENGTH as '0'.  If LENGTH is invalid, GNU 'troff' emits a
     warning in category 'number' and ignores the request.  The
     formatter's default title length is '6.5i'.  With no argument, the
     title length is restored to the previous value.  The title length
     is is associated with the environment (*note Environments::).

     The read-only register '.lt' interpolates the title line length.

 -- Request: .pc [char]
     Set the page number character to CHAR.  With no argument, the page
     number character is disabled.  'pc' does not affect the
     register '%'.

   The following example exercises title features.

     .lt 50n
     This is my partially collected
     .tl 'Isomers 2023'%'Dextrose Edition'
     line.
         => Isomers 2023             1        Dextrose Edition
         => This is my partially collected line.

   We most often see titles used in page header and footer traps.  *Note
Traps::.

   (1) *Note Traps::.

5.18 Page Control
=================

Discretionary page breaks can prevent the unwanted separation of
content.  A new page number takes effect during page ejection; see *note
The Implicit Page Trap::.

 -- Request: .bp [page-number]
 -- Request: .bp +page-number
 -- Request: .bp -page-number
 -- Register: \n[%]
     Break the page and change (increase or decrease) the next page
     number per the numeric expression PAGE-NUMBER.  If PAGE-NUMBER is
     invalid, GNU 'troff' emits a warning in category 'number' and
     ignores the argument.  This request causes a break.  A page break
     advances the vertical drawing position to the bottom of the page,
     springing traps.  *Note Page Location Traps::.  'bp' has effect
     only if invoked within the top-level diversion.(1)  (*note Page
     Control-Footnote-1::) This request is incorrectly documented in the
     AT&T 'troff' manual as having a default scaling unit of 'v'.

     The register '%' interpolates the current page number.

          .de BP
          '  bp \" schedule page break once current line is output
          ..

 -- Request: .ne [space]
     Force a page break if insufficient vertical space is available
     (assert "needed" space).  'ne' tests the distance to the next page
     location trap; see *note Page Location Traps::, and breaks the page
     if that amount is less than SPACE.  The default scaling unit is
     'v'.  If SPACE is invalid, GNU 'troff' emits a warning in category
     'number' and ignores the argument.  If SPACE is not specified, '1v'
     is assumed.

     We can require space for at least the first two output lines of a
     paragraph, preventing its first line from being widowed at the page
     bottom.

          .ne 2v
          Considering how common illness is,
          how tremendous the spiritual change that it brings,
          how astonishing,
          when the lights of health go down,
          the undiscovered countries that are then disclosed,
          what wastes and deserts of the soul a slight attack
          of influenza brings to view,

     This method is reliable only if no output line is pending when 'ne'
     is invoked.  When macro packages are used, this is often not the
     case: their paragraphing macros perform the break.  You may need to
     experiment with placing the 'ne' after the paragraphing macro, or
     'br' and 'ne' before it.

     'ne' is also useful to force grouping of section headings with
     their subsequent paragraphs, or tables with their captions and/or
     explanations.  Macro packages often use 'ne' with diversions to
     implement keeps and displays; see *note Diversions::.  They may
     also offer parameters for widow and orphan management.

 -- Request: .sv [space]
 -- Request: .os
     Require vertical space as 'ne' does, but also save it for later
     output by the 'os' request.  If SPACE is available before the next
     page location trap, it is output immediately.  Both requests ignore
     a partially collected line, taking effect at the next break.  'sv'
     and 'os' ignore no-space mode (recall *note Manipulating
     Spacing::).  While the 'sv' request allows negative values for
     SPACE, 'os' ignores them.  The default scaling unit is 'v'.  If
     SPACE is not specified, '1v' is assumed.

 -- Register: \n[nl]
     'nl' interpolates or sets the vertical drawing position.  When the
     formatter starts, the first page transition hasn't happened yet,
     and 'nl' is negative.  If a header trap has been planted on the
     page (typically at vertical position '0'), you can assign a
     negative value to 'nl' to spring it if that page has already
     started (*note Page Location Traps::).

          .de HD
          .  sp
          .  tl ''Goldbach Solution''
          .  sp
          ..
          .
          First page.
          .bp
          .wh 0 HD \" plant header trap at top of page
          .nr nl (-1)
          Second page.
              => First page.
              =>
              => (blank lines elided)
              =>
              =>                         Goldbach Solution
              =>
              => (blank lines elided)
              =>
              => Second page.

     Without resetting 'nl' to a negative value, the trap just planted
     would be active beginning with the _next_ page, not the current
     one.

     *Note Diversions::, for a comparison of 'nl' with the '.h' and '.d'
     registers.

   (1) *Note Diversions::.

5.19 Using Fonts
================

In digital typography, a "font" is a collection of characters in a
specific typeface that a device can render as glyphs at a desired
size.(1)  (*note Using Fonts-Footnote-1::) A 'roff' formatter can change
typefaces at any point in the text.  The basic faces are a set of
"styles" combining upright and slanted shapes with normal and heavy
stroke weights: 'R', 'I', 'B', and 'BI'--these stand for roman, italic,
bold, and bold-italic.  For linguistic text, GNU 'troff' groups
typefaces into "families" containing each of these styles.(2)  (*note
Using Fonts-Footnote-2::) A "text font" is thus often a family combined
with a style, but it need not be: consider the 'ps' and 'pdf' devices'
'ZCMI' (Zapf Chancery Medium italic)--often, no other style of Zapf
Chancery Medium is provided.  On typesetting devices, at least one
"special font" is available, comprising "unstyled" glyphs for
mathematical operators and other purposes.

   Like AT&T 'troff', GNU 'troff' does not itself load or manipulate a
digital font file;(3) (*note Using Fonts-Footnote-3::) instead it works
with a "font description file" that characterizes it, including its
glyph repertoire and the "metrics" (dimensions) of each glyph.(4)
(*note Using Fonts-Footnote-4::) This information permits the formatter
to accurately place glyphs with respect to each other.  Before using a
font description, the formatter associates it with a "mounting
position", a place in an ordered list of available typefaces.  So that a
document need not be strongly coupled to a specific font family, in GNU
'troff' an output device can associate a style in the abstract sense
with a mounting position.  Thus the default family can be combined with
a style dynamically, producing a "resolved font name".

   Fonts often have trademarked names, and even Free Software fonts can
require renaming upon modification.  'groff' maintains a convention that
a device's serif font family is given the name 'T' ("Times"), its
sans-serif family 'H' ("Helvetica"), and its monospaced family 'C'
("Courier").  Historical inertia has driven 'groff''s font identifiers
to short uppercase abbreviations of font names, as with 'TR', 'TI',
'TB', 'TBI', and a special font 'S'.

   The default family used with abstract styles can be changed at any
time; initially, it is 'T'.  Typically, abstract styles are arranged in
the first four mounting positions in the order shown above.  The default
mounting position, and therefore style, is always '1' ('R').  By issuing
appropriate formatter instructions, you can override these defaults
before your document writes its first glyph.

   Terminal output devices cannot change font families and lack special
fonts.  They support style changes by overstriking, or by altering
ISO 6429/ECMA-48 "graphic renditions" (character cell attributes).

   (1) Terminals and some output devices have fonts that render at only
one or two sizes.  As examples of the latter, take the 'groff' 'lj4'
device's Lineprinter, and 'lbp''s Courier and Elite faces.

   (2) Font designers prepare families such that the styles share
esthetic properties.

   (3) Historically, the fonts 'troff's dealt with were not Free
Software or, as with the Graphic Systems C/A/T, did not even exist in
the digital domain.

   (4) *Note Font Description File Format::.

5.19.1 Selecting Fonts
----------------------

We use "font" to refer to any of several means of identifying a font: by
mounting position ('3'), by abstract style ('B'), or by its identifier
('TB').

 -- Request: .ft [font]
 -- Escape sequence: \ff
 -- Escape sequence: \f(fn
 -- Escape sequence: \f[font]
 -- Register: \n[.fn]
     The 'ft' request selects the typeface FONT.  If the argument is
     absent or 'P', it selects the previously chosen font.  If FONT is a
     non-negative integer, it is interpreted as mounting position; the
     font mounted there is selected.  If that position refers to an
     abstract style, it is combined with the default family (see 'fam'
     and '\F' below) to make a resolved font name.  If the mounting
     position is not a style and no font is mounted there, GNU 'troff'
     emits a warning in category 'font' and ignores the request.

     If FONT matches a style name, it is combined with the current
     family to make a resolved font name.  Otherwise, FONT is assumed to
     already be a resolved font name.

     The resolved font name is subject to translation (see request 'ftr'
     below).  Next, the (possibly translated) font name's mounting
     position is looked up; if not mounted, FONT is sought on the file
     system as a font description file and, if located, automatically
     mounted at the next available position (see register '.fp' below).
     If the font was mounted using an identifier different from its font
     description file name (see request 'fp' below), that file name is
     then looked up.  If a font description file for the resolved font
     name is not found, GNU 'troff' emits a warning in category 'font'
     and ignores the request.

     The '\f' escape sequence is similar, using one-character name (or
     mounting position) F, two-character name FN, or a name FONT of
     arbitrary length.  '\f[]' selects the previous font.  The syntax
     form '\fP' is supported for backward compatibility, and '\f[P]' for
     consistency.

          eggs, bacon,
          .ft I
          spam,
          .ft
          and sausage.
          .br
          eggs, bacon, \fIspam,\fP and sausage.
              => eggs, bacon, spam, and sausage
              => eggs, bacon, spam, and sausage

     The current and previously selected fonts are properties of the
     environment (*note Environments::).

     The read-only string-valued register '.fn' contains the resolved
     font name of the selected font.

     '\f' doesn't produce an input token in GNU 'troff'; it thus can be
     used in requests that expect a single-character argument.  We can
     assign a font to a margin character as follows (*note
     Miscellaneous::).

          .mc \f[I]x\f[]

 -- Request: .ftr f [g]
     Translate font F to font G.  Whenever a font named F is referred to
     in a '\f' escape sequence, in the 'F' and 'S' conditional
     operators, or in the 'ft', 'ul', 'bd', 'cs', 'tkf', 'special',
     'fspecial', 'fp', or 'sty' requests, font G is used.  If G is
     missing or equal to F the translation is undone.

     Font translations cannot be chained.

          .ftr XXX TR
          .ftr XXX YYY
          .ft XXX
              error-> warning: can't find font 'XXX'

 -- Request: .fzoom f [zoom]
 -- Register: \n[.zoom]
     Set magnification of font F to factor ZOOM, which must be a
     non-negative integer multiple of 1/1000th.  This request is useful
     to adjust the optical size of a font in relation to the others.  In
     the example below, font 'CR' is magnified by 10% (the zoom factor
     is thus 1.1).

          .fam P
          .fzoom CR 1100
          .ps 12
          Palatino and \f[CR]Courier\f[]

     A missing or zero value of ZOOM is the same as a value of 1000,
     which means no magnification.  F must be a resolved font name, not
     an abstract style.

     The magnification of a font is completely transparent to GNU
     'troff'; a change of the zoom factor doesn't cause any effect
     except that the dimensions of glyphs, (word) spaces, kerns, etc.,
     of the affected font are adjusted accordingly.

     The zoom factor of the current font is available in the read-only
     register '.zoom', in multiples of 1/1000th.  It returns zero if
     there is no magnification.

5.19.2 Font Families
--------------------

To accommodate the wide variety of fonts available, GNU 'troff'
distinguishes "font families" and "font styles".  A resolved font name
is the catenation of a font family and a style.  Selecting an abstract
style causes GNU 'troff' to combine it with the default font family.

   You can thus compose a document using abstract styles exclusively for
its body or running text, selecting a specific family only for titles or
examples, for instance, and change the default family on the command
line (recall *note Groff Options::).

   Fonts for the devices 'ps', 'pdf', 'dvi', 'lj4', 'lbp', and the X11
devices support this mechanism.  By default, GNU 'troff' uses the Times
family with the four styles 'R', 'I', 'B', and 'BI'.

 -- Request: .fam [family]
 -- Register: \n[.fam]
 -- Escape sequence: \Ff
 -- Escape sequence: \F(fm
 -- Escape sequence: \F[family]
     Set the default font family, used in combination with abstract
     styles to construct a resolved font name, to FAMILY (one-character
     name F, two-character name FM).  If no argument is given, GNU
     'troff' selects the previous font family; if there none, is it
     falls back to the device's default(1) (*note Font
     Families-Footnote-1::) or its own ('T').

     The '\F' escape sequence works similarly.  In disanalogy to '\f',
     '\FP' makes 'P' the default family.  Use '\F[]' to select the
     previous default family.  The default font family is available in
     the read-only string-valued register '.fam'; it is associated with
     the environment (*note Environments::).

          spam,     \" startup defaults are T (Times) R (roman)
          .fam H    \" make Helvetica the default family
          spam,     \" family H + style R = HR
          .ft B     \" family H + style B = HB
          spam,
          .ft CR    \" Courier roman (default family not changed)
          spam,
          .ft       \" back to Helvetica bold
          spam,
          .fam T    \" make Times the default family
          spam,     \" family T + style B = TB
          .ft AR    \" font AR (not a style)
          baked beans,
          .ft R     \" family T + style R = TR
          and spam.

     '\F' doesn't produce an input token in GNU 'troff'.  As a
     consequence, it can be used in requests like 'mc' (which expects a
     single character as an argument) to change the font family on the
     fly.

          .mc \F[P]x\F[]

 -- Request: .sty n style
 -- Register: \n[.sty]
     Associate an abstract style STYLE with mounting position N, which
     must be a non-negative integer.  If the requests 'cs', 'bd', 'tkf',
     'uf', or 'fspecial' are applied to an abstract style, they are
     instead applied to the member of the current family corresponding
     to that style.

     The default family can be set with the '-f' option (*note Groff
     Options::).  The 'styles' command in the 'DESC' file controls which
     font positions (if any) are initially associated with abstract
     styles rather than fonts.

     *Caution:* The STYLE argument is not validated.  Errors may occur
     later, when the formatter attempts to construct a resolved font
     name, or format a character for output.

          .nr BarPos \n[.fp]
          .sty \n[.fp] Bar
          .fam Foo
          .ft \n[BarPos]
          .tm .f=\n[.f]
          A
              error-> error: no font family named 'Foo' exists
              error-> .f=41
              error-> error: cannot format glyph: no current font

     When an abstract style has been selected, the read-only
     string-valued register '.sty' interpolates its name; this datum is
     associated with the environment (*note Environments::).  Otherwise,
     '.sty' interpolates nothing.

   (1) *Note DESC File Format::.

5.19.3 Font Positions
---------------------

To support typeface indirection through abstract styles, and for
compatibility with AT&T 'troff', the formatter maintains a list of font
"positions" at which fonts required by a document are "mounted".  An
output device's description file 'DESC' typically configures a set of
pre-mounted fonts; see *note Device and Font Description Files::.  A
font need not be explicitly mounted before it is selected; GNU 'troff'
will search 'GROFF_FONT_PATH' for it by name and mount it at the first
free mounting position on demand.

 -- Request: .fp pos id [font-description-file-name]
 -- Register: \n[.f]
 -- Register: \n[.fp]
     Mount a font under the name ID at mounting position POS, a
     non-negative integer.  When the formatter starts up, it reads the
     output device's description to mount an initial set of faces, and
     selects font position 1.  Position 0 is unused by default.  Unless
     the FONT-DESCRIPTION-FILE-NAME argument is given, ID should be the
     name of a font description file stored in a directory corresponding
     to the selected output device.  GNU 'troff' does not traverse
     directories to locate the font description file.

     The optional third argument enables font names to be aliased, which
     can be necessary in compatibility mode since AT&T 'troff' syntax
     affords no means of identifying fonts with names longer than two
     characters, like 'TBI' or 'ZCMI', in a font selection escape
     sequence.  *Note Compatibility Mode::.  You can also alias fonts on
     mounting for convenience or abstraction.  (See below regarding the
     '.fp' register.)

          .fp \n[.fp] SC ZCMI
          Send a \f(SChand-written\fP thank-you note.
          .fp \n[.fp] Emph TI
          .fp \n[.fp] Strong TB
          Are \f[Emph]these names\f[] \f[Strong]comfortable\f[]?

     'DESC', 'P', and non-negative integers are not usable as font
     identifiers.

     The position of the currently selected font (or abstract style) is
     available in the read-only register '.f'.  It is associated with
     the environment (*note Environments::).

     You can copy the value of '.f' to another register to save it for
     later use.

          .nr saved-font \n[.f]
          ... text involving many font changes ...
          .ft \n[saved-font]

     The index of the next (non-zero) free font position is available in
     the read-only register '.fp'.  Fonts not listed in the 'DESC' file
     are automatically mounted at position '\n[.fp]' when selected with
     the 'ft' request or '\f' escape sequence.  When mounting a font at
     a position explicitly with the 'fp' request, this same practice
     should be followed, although GNU 'troff' does not enforce this
     strictly.

5.19.4 Using Symbols
--------------------

A "glyph" is a graphical representation of a "character".  While a
character is an abstraction of semantic information, a glyph is
something that can be seen on screen or paper.  A character has many
possible representation forms (for example, the character 'A' can be
written in an upright or slanted typeface, producing distinct glyphs).
Sometimes, a sequence of characters map to a single glyph: this is a
"ligature"--the most common is 'fi'.

   Space characters never become glyphs in GNU 'troff'.  If not
discarded (as when trailing on text lines), they are represented by
horizontal motions in the output.

   A "symbol" is simply a named glyph.  Within 'gtroff', all glyph names
of a particular font are defined in its font file.  If the user requests
a glyph not available in this font, 'gtroff' looks up an ordered list of
"special fonts".  By default, the PostScript output device supports the
two special fonts 'SS' (slanted symbols) and 'S' (symbols) (the former
is looked up before the latter).  Other output devices use different
names for special fonts.  Fonts mounted with the 'fonts' keyword in the
'DESC' file are globally available.  To install additional special fonts
locally (i.e., for a particular font), use the 'fspecial' request.

   Here are the exact rules how 'gtroff' searches a given symbol:

   * If the symbol has been defined with the 'char' request, use it.
     This hides a symbol with the same name in the current font.

   * Check the current font.

   * If the symbol has been defined with the 'fchar' request, use it.

   * Check whether the current font has a font-specific list of special
     fonts; test all fonts in the order of appearance in the last
     'fspecial' call if appropriate.

   * If the symbol has been defined with the 'fschar' request for the
     current font, use it.

   * Check all fonts in the order of appearance in the last 'special'
     call.

   * If the symbol has been defined with the 'schar' request, use it.

   * As a last resort, consult all fonts loaded up to now for special
     fonts and check them, starting with the lowest font number.  This
     can sometimes lead to surprising results since the 'fonts' line in
     the 'DESC' file often contains empty positions, which are filled
     later on.  For example, consider the following:

          fonts 3 0 0 FOO

     This mounts font 'foo' at font position 3.  We assume that 'FOO' is
     a special font, containing glyph 'foo', and that no font has been
     loaded yet.  The line

          .fspecial BAR BAZ

     makes font 'BAZ' special only if font 'BAR' is active.  We further
     assume that 'BAZ' is really a special font, i.e., the font
     description file contains the 'special' keyword, and that it also
     contains glyph 'foo' with a special shape fitting to font 'BAR'.
     After executing 'fspecial', font 'BAR' is loaded at font
     position 1, and 'BAZ' at position 2.

     We now switch to a new font 'XXX', trying to access glyph 'foo'
     that is assumed to be missing.  There are neither font-specific
     special fonts for 'XXX' nor any other fonts made special with the
     'special' request, so 'gtroff' starts the search for special fonts
     in the list of already mounted fonts, with increasing font
     positions.  Consequently, it finds 'BAZ' before 'FOO' even for
     'XXX', which is not the intended behaviour.

   *Note Device and Font Description Files::, and *note Special Fonts::,
for more details.

   The 'groff_char(7)' man page houses a complete list of predefined
special character names, but the availability of any as a glyph is
device- and font-dependent.  For example, say

     man -Tdvi groff_char > groff_char.dvi

to obtain those available with the DVI device and default font
configuration.(1)  (*note Using Symbols-Footnote-1::) If you want to use
an additional macro package to change the fonts used, 'groff' (or
'gtroff') must be run directly.

     groff -Tdvi -mec -man groff_char.7 > groff_char.dvi

   Special character names not listed in 'groff_char(7)' are derived
algorithmically, using a simplified version of the Adobe Glyph List
(AGL) algorithm, which is described in
<https://github.com/adobe-type-tools/agl-aglfn>.  The (frozen) set of
names that can't be derived algorithmically is called the "'groff' glyph
list (GGL)".

   * A glyph for Unicode character U+XXXX[X[X]], which is not a
     composite character is named 'uXXXX[X[X]]'.  X must be an uppercase
     hexadecimal digit.  Examples: 'u1234', 'u008E', 'u12DB8'.  The
     largest Unicode value is 0x10FFFF. There must be at least four 'X'
     digits; if necessary, add leading zeroes (after the 'u').  No zero
     padding is allowed for character codes greater than 0xFFFF.
     Surrogates (i.e., Unicode values greater than 0xFFFF represented
     with character codes from the surrogate area U+D800-U+DFFF) are not
     allowed either.

   * A glyph representing more than a single input character is named

          'u' COMPONENT1 '_' COMPONENT2 '_' COMPONENT3 ...

     Example: 'u0045_0302_0301'.

     For simplicity, all Unicode characters that are composites must be
     maximally decomposed to NFD;(2) (*note Using Symbols-Footnote-2::)
     for example, 'u00CA_0301' is not a valid glyph name since U+00CA
     (LATIN CAPITAL LETTER E WITH CIRCUMFLEX) can be further decomposed
     into U+0045 (LATIN CAPITAL LETTER E) and U+0302 (COMBINING
     CIRCUMFLEX ACCENT).  'u0045_0302_0301' is thus the glyph name for
     U+1EBE, LATIN CAPITAL LETTER E WITH CIRCUMFLEX AND ACUTE.

   * groff maintains a table to decompose all algorithmically derived
     glyph names that are composites itself.  For example, 'u0100'
     (LATIN LETTER A WITH MACRON) is automatically decomposed into
     'u0041_0304'.  Additionally, a glyph name of the GGL is preferred
     to an algorithmically derived glyph name; 'groff' also
     automatically does the mapping.  Example: The glyph 'u0045_0302' is
     mapped to '^E'.

   * glyph names of the GGL can't be used in composite glyph names; for
     example, '^E_u0301' is invalid.

 -- Escape sequence: \(nm
 -- Escape sequence: \[name]
 -- Escape sequence: \[base-glyph combining-component ...]
     Typeset a special character NAME (two-character name NM) or a
     composite glyph consisting of BASE-GLYPH overlaid with one or more
     COMBINING-COMPONENTs.  For example, '\[A ho]' is a capital letter
     "A" with a "hook accent" (ogonek).

     There is no special syntax for one-character names--the analogous
     form '\N' would collide with other escape sequences.  However, the
     four escape sequences '\'', '\-', '\_', and '\`', are translated on
     input to the special character escape sequences '\[aa]', '\[-]',
     '\[ul]', and '\[ga]', respectively.

     A special character name of length one is not the same thing as an
     ordinary character: that is, the character 'a' is not the same as
     '\[a]'.

     If NAME is undefined, a warning in category 'char' is produced and
     the escape is ignored.  *Note Warnings::, for information about the
     enablement and suppression of warnings.

     GNU 'troff' resolves '\[...]' with more than a single component as
     follows:

        * Any component that is found in the GGL is converted to the
          'uXXXX' form.

        * Any component 'uXXXX' that is found in the list of
          decomposable glyphs is decomposed.

        * The resulting elements are then concatenated with '_' in
          between, dropping the leading 'u' in all elements but the
          first.

     No check for the existence of any component (similar to 'tr'
     request) is done.

     Examples:

     '\[A ho]'
          'A' maps to 'u0041', 'ho' maps to 'u02DB', thus the final
          glyph name would be 'u0041_02DB'.  This is not the expected
          result: the ogonek glyph 'ho' is a spacing ogonek, but for a
          proper composite a non-spacing ogonek (U+0328) is necessary.
          Looking into the file 'composite.tmac', one can find
          '.composite ho u0328', which changes the mapping of 'ho' while
          a composite glyph name is constructed, causing the final glyph
          name to be 'u0041_0328'.

     '\[^E u0301]'
     '\[^E aa]'
     '\[E a^ aa]'
     '\[E ^ ']'
          '^E' maps to 'u0045_0302', thus the final glyph name is
          'u0045_0302_0301' in all forms (assuming proper calls of the
          'composite' request).

     It is not possible to define glyphs with names like 'A ho' within a
     'groff' font file.  This is not really a limitation; instead, you
     have to define 'u0041_0328'.

 -- Escape sequence: \C'xxx'
     Typeset the glyph of the special character XXX.  Normally, it is
     more convenient to use '\[XXX]', but '\C' has some advantages: it
     is compatible with AT&T device-independent 'troff' (and therefore
     available in compatibility mode(3) (*note Using
     Symbols-Footnote-3::)) and can interpolate special characters with
     ']' in their names.  The delimiter need not be a neutral
     apostrophe; see *note Delimiters::.

 -- Request: .composite id1 id2
     Map special character name ID1 to ID2 if ID1 is used in '\[...]'
     with more than one component.  See above for examples.  This is a
     strict rewriting of the special character name; no check is
     performed for the existence of a glyph for either.  A set of
     default mappings for many accents can be found in the file
     'composite.tmac', loaded by the default 'troffrc' at startup.

 -- Escape sequence: \N'n'
     Typeset the glyph with code N in the current font ('n' is _not_ the
     input character code).  The number N can be any non-negative
     decimal integer.  Most devices only have glyphs with codes between
     0 and 255; the Unicode output device uses codes in the range
     0-65535.  If the current font does not contain a glyph with that
     code, special fonts are _not_ searched.  The '\N' escape sequence
     can be conveniently used in conjunction with the 'char' request:

          .char \[phone] \f[ZD]\N'37'

     The code of each glyph is given in the fourth column in the font
     description file after the 'charset' command.  It is possible to
     include unnamed glyphs in the font description file by using a name
     of '---'; the '\N' escape sequence is the only way to use these.

     No kerning is applied to glyphs accessed with '\N'.  The delimiter
     need not be a neutral apostrophe; see *note Delimiters::.

   A few escape sequences are also special characters.

 -- Escape sequence: \'
     An escaped neutral apostrophe is a synonym for '\[aa]' (acute
     accent).

 -- Escape sequence: \`
     An escaped grave accent is a synonym for '\[ga]' (grave accent).

 -- Escape sequence: \-
     An escaped hyphen-minus is a synonym for '\[-]' (minus sign).

 -- Escape sequence: \_
     An escaped underscore ("low line") is a synonym for '\[ul]'
     (underrule).  On typesetting devices, the underrule is
     font-invariant and drawn lower than the underscore '_'.

 -- Request: .cflags n c1 c2 ...
     Assign properties encoded by the number N to characters C1, C2, and
     so on.

     Input characters, including special characters introduced by an
     escape, have certain properties associated with them.(4)  (*note
     Using Symbols-Footnote-4::) These properties can be modified with
     this request.  The first argument is the sum of the desired flags
     and the remaining arguments are the characters to be assigned those
     properties.  Spaces between the CN arguments are optional.  Any
     argument CN can be a character class defined with the 'class'
     request rather than an individual character.  *Note Character
     Classes::.

     The non-negative integer N is the sum of any of the following.
     Some combinations are nonsensical, such as '33' (1 + 32).

     '1'
          Recognize the character as ending a sentence if followed by a
          newline or two spaces.  Initially, characters '.?!' have this
          property.

     '2'
          Enable breaks before the character.  A line is not broken at a
          character with this property unless the characters on each
          side both have non-zero hyphenation codes.  This exception can
          be overridden by adding 64.  Initially, no characters have
          this property.

     '4'
          Enable breaks after the character.  A line is not broken at a
          character with this property unless the characters on each
          side both have non-zero hyphenation codes.  This exception can
          be overridden by adding 64.  Initially, characters
          '\-\[hy]\[em]' have this property.

     '8'
          Mark the glyph associated with this character as overlapping
          other instances of itself horizontally.  Initially, characters
          '\[ul]\[rn]\[ru]\[radicalex]\[sqrtex]' have this property.

     '16'
          Mark the glyph associated with this character as overlapping
          other instances of itself vertically.  Initially, the
          character '\[br]' has this property.

     '32'
          Mark the character as transparent for the purpose of
          end-of-sentence recognition.  In other words, an
          end-of-sentence character followed by any number of characters
          with this property is treated as the end of a sentence if
          followed by a newline or two spaces.  This is the same as
          having a zero space factor in TeX.  Initially, characters
          '"')]*\[dg]\[dd]\[rq]\[cq]' have this property.

     '64'
          Ignore hyphenation codes of the surrounding characters.  Use
          this in combination with values 2 and 4 (initially, no
          characters have this property).

          For example, if you need an automatic break point after the
          en-dash in numeric ranges like "3000-5000", insert

               .cflags 68 \[en]

          into your document.  However, this practice can lead to bad
          layout if done thoughtlessly; in most situations, a better
          solution instead of changing the 'cflags' value is to insert
          '\:' right after the hyphen at the places that really need a
          break point.

     The remaining values were implemented for East Asian language
     support; those who use alphabetic scripts exclusively can disregard
     them.

     '128'
          Prohibit a line break before the character, but allow a line
          break after the character.  This works only in combination
          with flags 256 and 512 and has no effect otherwise.
          Initially, no characters have this property.

     '256'
          Prohibit a line break after the character, but allow a line
          break before the character.  This works only in combination
          with flags 128 and 512 and has no effect otherwise.
          Initially, no characters have this property.

     '512'
          Allow line break before or after the character.  This works
          only in combination with flags 128 and 256 and has no effect
          otherwise.  Initially, no characters have this property.

     In contrast to values 2 and 4, the values 128, 256, and 512 work
     pairwise.  If, for example, the left character has value 512, and
     the right character 128, no break will be automatically inserted
     between them.  If we use value 6 instead for the left character, a
     break after the character can't be suppressed since the neighboring
     character on the right doesn't get examined.

 -- Request: .char c [contents]
 -- Request: .fchar c [contents]
 -- Request: .fschar f c [contents]
 -- Request: .schar c [contents]
     Define a new character or glyph C to be CONTENTS, which can be
     empty.  More precisely, 'char' defines a 'groff' object (or
     redefines an existing one) that is accessed with the name C on
     input, and produces CONTENTS on output.  Every time glyph C needs
     to be printed, CONTENTS is processed in a temporary environment and
     the result is wrapped up into a single object.  Compatibility mode
     is turned off and the escape character is set to '\' while CONTENTS
     is processed.  Any emboldening, constant spacing, or track kerning
     is applied to this object rather than to individual glyphs in
     CONTENTS.

     An object defined by these requests can be used just like a normal
     glyph provided by the output device.  In particular, other
     characters can be translated to it with the 'tr' or 'trin'
     requests; it can be made the leader character with the 'lc'
     request; repeated patterns can be drawn with it using the '\l' and
     '\L' escape sequences; and words containing C can be hyphenated
     correctly if the 'hcode' request is used to give the object a
     hyphenation code.

     There is a special anti-recursion feature: use of the object within
     its own definition is handled like a normal character (not defined
     with 'char').

     The 'tr' and 'trin' requests take precedence if 'char' accesses the
     same symbol.

          .tr XY
          X
              => Y
          .char X Z
          X
              => Y
          .tr XX
          X
              => Z

     The 'fchar' request defines a fallback glyph: 'gtroff' only checks
     for glyphs defined with 'fchar' if it cannot find the glyph in the
     current font.  'gtroff' carries out this test before checking
     special fonts.

     'fschar' defines a fallback glyph for font F: 'gtroff' checks for
     glyphs defined with 'fschar' after the list of fonts declared as
     font-specific special fonts with the 'fspecial' request, but before
     the list of fonts declared as global special fonts with the
     'special' request.

     Finally, the 'schar' request defines a global fallback glyph:
     'gtroff' checks for glyphs defined with 'schar' after the list of
     fonts declared as global special fonts with the 'special' request,
     but before the already mounted special fonts.

     *Note Character Classes::.

 -- Request: .rchar c ...
 -- Request: .rfschar f c ...
     Remove definition of each ordinary or special character C, undoing
     the effect of a 'char', 'fchar', or 'schar' request.  Those
     supplied by font description files cannot be removed.  Spaces and
     tabs may separate C arguments.

     The request 'rfschar' removes glyph definitions defined with
     'fschar' for font F.

   (1) Not all versions of the 'man' program support the '-T' option;
use the subsequent example for an alternative.

   (2) This is "Normalization Form D" as documented in Unicode Standard
Annex #15 (<https://unicode.org/reports/tr15/>).

   (3) *Note Compatibility Mode::.

   (4) Output glyphs don't--to GNU 'troff', a glyph is simply a box with
an index into a font, a given height above and depth below the baseline,
and a width.

5.19.5 Character Classes
------------------------

Classes are particularly useful for East Asian languages such as
Chinese, Japanese, and Korean, where the number of needed characters is
much larger than in European languages, and where large sets of
characters share the same properties.

 -- Request: .class name c1 c2 ...
     Define a character class (or simply "class") NAME comprising the
     characters C1, C2, and so on.

     A class thus defined can then be referred to in lieu of listing all
     the characters within it.  Currently, only the 'cflags' request can
     handle references to character classes.

     In the request's simplest form, each CN is a character (or special
     character).

          .class [quotes] ' \[aq] \[dq] \[oq] \[cq] \[lq] \[rq]

     Since class and glyph names share the same name space, it is
     recommended to start and end the class name with '[' and ']',
     respectively, to avoid collisions with existing character names
     defined by GNU 'troff' or the user (with 'char' and related
     requests).  This practice applies the presence of ']' in the class
     name to prevent the use of the special character escape form
     '\[...]', thus you must use the '\C' escape to access a class with
     such a name.

     You can also use a character range notation consisting of a start
     character followed by '-' and then an end character.  Internally,
     GNU 'troff' converts these two symbol names to Unicode code points
     (according to the 'groff' glyph list [GGL]), which then give the
     start and end value of the range.  If that fails, the class
     definition is skipped.

     Furthermore, classes can be nested.

          .class [prepunct] , : ; > }
          .class [prepunctx] \C'[prepunct]' \[u2013]-\[u2016]

     The class '[prepunctx]' thus contains the contents of the class
     '[prepunct]' as defined above (the set ', : ; > }'), and characters
     in the range between 'U+2013' and 'U+2016'.

     If you want to include '-' in a class, it must be the first
     character value in the argument list, otherwise it gets
     misinterpreted as part of the range syntax.

     It is not possible to use class names as end points of range
     definitions.

     A typical use of the 'class' request is to control line-breaking
     and hyphenation rules as defined by the 'cflags' request.  For
     example, to inhibit line breaks before the characters belonging to
     the 'prepunctx' class defined in the previous example, you can
     write the following.

          .cflags 2 \C'[prepunctx]'

     See the 'cflags' request in *note Using Symbols::, for more
     details.

5.19.6 Special Fonts
--------------------

Special fonts are those that 'gtroff' searches when it cannot find the
requested glyph in the current font.  The Symbol font is usually a
special font.

   'gtroff' provides the following two requests to add more special
fonts.  *Note Using Symbols::, for a detailed description of the glyph
searching mechanism in 'gtroff'.

   Usually, only non-TTY devices have special fonts.

 -- Request: .special [s1 s2 ...]
 -- Request: .fspecial f [s1 s2 ...]
     Use the 'special' request to define special fonts.  Initially, this
     list is empty.

     Use the 'fspecial' request to designate special fonts only when
     font F is active.  Initially, this list is empty.

     Previous calls to 'special' or 'fspecial' are overwritten; without
     arguments, the particular list of special fonts is set to empty.
     Special fonts are searched in the order they appear as arguments.

     All fonts that appear in a call to 'special' or 'fspecial' are
     loaded.

     *Note Using Symbols::, for the exact search order of glyphs.

5.19.7 Artificial Fonts
-----------------------

There are a number of requests and escape sequences for artificially
creating fonts.  These are largely vestiges of the days when output
devices did not have a wide variety of fonts, and when 'nroff' and
'troff' were separate programs.  Most of them are no longer necessary in
GNU 'troff'.  Nevertheless, they are supported.

 -- Escape sequence: \H'height'
 -- Escape sequence: \H'+height'
 -- Escape sequence: \H'-height'
 -- Register: \n[.height]
     Change (increment, decrement) the height of the current font, but
     not the width.  If HEIGHT is zero, restore the original height.
     Default scaling unit is 'z'.

     The read-only register '.height' contains the font height as set by
     '\H'.

     Currently, only the '-Tps' and '-Tpdf' devices support this
     feature.

     '\H' doesn't produce an input token in GNU 'troff'.  As a
     consequence, it can be used in requests like 'mc' (which expects a
     single character as an argument) to change the font on the fly:

          .mc \H'+5z'x\H'0'

     In compatibility mode, 'gtroff' behaves differently: If an
     increment or decrement is used, it is always taken relative to the
     current type size and not relative to the previously selected font
     height.  Thus,

          .cp 1
          \H'+5'test \H'+5'test

     prints the word 'test' twice with the same font height (five points
     larger than the current font size).

 -- Escape sequence: \S'slant'
 -- Register: \n[.slant]
     Slant the current font by SLANT degrees.  Positive values slant to
     the right.  Only integer values are possible.

     The read-only register '.slant' contains the font slant as set by
     '\S'.

     Currently, only the '-Tps' and '-Tpdf' devices support this
     feature.

     '\S' doesn't produce an input token in GNU 'troff'.  As a
     consequence, it can be used in requests like 'mc' (which expects a
     single character as an argument) to change the font on the fly:

          .mc \S'20'x\S'0'

     This escape is incorrectly documented in the AT&T 'troff' manual;
     the slant is always set to an absolute value.

 -- Request: .ul [lines]
     The 'ul' request normally underlines subsequent lines if a TTY
     output device is used.  Otherwise, the lines are printed in italics
     (only the term 'underlined' is used in the following).  The single
     argument is the quantity of input lines to be underlined; with no
     argument, the next line is underlined.  If LINES is zero or
     negative, stop the effects of 'ul' (if it was active).  Requests
     and empty lines do not count for computing the number of underlined
     input lines, even if they produce some output like 'tl'.  Lines
     inserted by macros (e.g., invoked by a trap) do count.

     At the beginning of 'ul', the current font is stored and the
     underline font is activated.  Within the span of a 'ul' request, it
     is possible to change fonts, but after the last line affected by
     'ul' the saved font is restored.

     This number of lines still to be underlined is associated with the
     environment (*note Environments::).  The underline font can be
     changed with the 'uf' request.

     The 'ul' request does not underline spaces.

 -- Request: .cu [lines]
     The 'cu' request is similar to 'ul' but underlines spaces as well
     (if a TTY output device is used).

 -- Request: .uf font
     Set the underline font (globally) used by 'ul' and 'cu'.  By
     default, this is the font at position 2.  FONT can be either a
     non-negative font position or the name of a font.

 -- Request: .bd font [offset]
 -- Request: .bd font1 font2 [offset]
 -- Register: \n[.b]
     Embolden FONT by overstriking its glyphs offset by OFFSET units
     minus one.

     Two syntax forms are available.

        * Imitate a bold font unconditionally.  The first argument
          specifies the font to embolden, and the second is the number
          of basic units, minus one, by which the two glyphs are offset.
          If the second argument is missing, emboldening is turned off.

          FONT can be either a non-negative font position or the name of
          a font.

          OFFSET is available in the '.b' read-only register if a
          special font is active; in the 'bd' request, its default unit
          is 'u'.

        * Imitate a bold form conditionally.  Embolden FONT1 by OFFSET
          only if font FONT2 is the current font.  This request can be
          issued repeatedly to set up different emboldening values for
          different current fonts.  If the second argument is missing,
          emboldening is turned off for this particular current font.

          This affects special fonts only (either set up with the
          'special' command in font files or with the 'fspecial'
          request).

 -- Request: .cs font [width [em-size]]
     Switch to and from "constant glyph space mode".  If activated, the
     width of every glyph is WIDTH/36 ems.  The em size is given
     absolutely by EM-SIZE; if this argument is missing, the em value is
     taken from the current font size (as set with the 'ps' request)
     when the font is effectively in use.  Without second and third
     argument, constant glyph space mode is deactivated.

     Default scaling unit for EM-SIZE is 'z'; WIDTH is an integer.

5.19.8 Ligatures and Kerning
----------------------------

Ligatures are groups of characters that are run together, i.e, producing
a single glyph.  For example, the letters 'f' and 'i' can form a
ligature 'fi' as in the word 'file'.  This produces a cleaner look
(albeit subtle) to the printed output.  Usually, ligatures are not
available in fonts for TTY output devices.

   Most PostScript fonts support the fi and fl ligatures.  The C/A/T
typesetter that was the target of AT&T 'troff' also supported 'ff',
'ffi', and 'ffl' ligatures.  Advanced typesetters or 'expert' fonts may
include ligatures for 'ft' and 'ct', although GNU 'troff' does not
support these (yet).

   Only the current font is checked for ligatures and kerns; neither
special fonts nor special charcters defined with the 'char' request (and
its siblings) are taken into account.

 -- Request: .lg [flag]
 -- Register: \n[.lg]
     Switch the ligature mechanism on or off; if the parameter is
     non-zero or missing, ligatures are enabled, otherwise disabled.
     Default is on.  The current ligature mode can be found in the
     read-only register '.lg' (set to 1 or 2 if ligatures are enabled,
     0 otherwise).

     Setting the ligature mode to 2 enables the two-character ligatures
     (fi, fl, and ff) and disables the three-character ligatures (ffi
     and ffl).

   "Pairwise kerning" is another subtle typesetting mechanism that
modifies the distance between a glyph pair to improve readability.  In
most cases (but not always) the distance is decreased.  Typewriter-like
fonts and fonts for terminals where all glyphs have the same width don't
use kerning.

 -- Request: .kern [flag]
 -- Register: \n[.kern]
     Switch kerning on or off.  If the parameter is non-zero or missing,
     enable pairwise kerning, otherwise disable it.  The read-only
     register '.kern' is set to 1 if pairwise kerning is enabled,
     0 otherwise.

     If the font description file contains pairwise kerning information,
     glyphs from that font are kerned.  Kerning between two glyphs can
     be inhibited by placing '\&' between them: 'V\&A'.

     *Note Font Description File Format::.

   "Track kerning" expands or reduces the space between glyphs.  This
can be handy, for example, if you need to squeeze a long word onto a
single line or spread some text to fill a narrow column.  It must be
used with great care since it is usually considered bad typography if
the reader notices the effect.

 -- Request: .tkf f s1 n1 s2 n2
     Enable track kerning for font F.  If the current font is F the
     width of every glyph is increased by an amount between N1 and N2
     (N1, N2 can be negative); if the current type size is less than or
     equal to S1 the width is increased by N1; if it is greater than or
     equal to S2 the width is increased by N2; if the type size is
     greater than or equal to S1 and less than or equal to S2 the
     increase in width is a linear function of the type size.

     The default scaling unit is 'z' for S1 and S2, 'p' for N1 and N2.

     The track kerning amount is added even to the rightmost glyph in a
     line; for large values it is thus recommended to increase the line
     length by the same amount to compensate.

5.19.9 Italic Corrections
-------------------------

When typesetting adjacent glyphs from typefaces of different slants, the
space between them may require adjustment.

 -- Escape sequence: \/
     Apply an "italic correction": modify the spacing of the preceding
     glyph so that the distance between it and the following glyph is
     correct if the latter is of upright shape.  For example, if an
     italic 'f' is followed immediately by a roman right parenthesis,
     then in many fonts the top right portion of the 'f' overlaps the
     top left of the right parenthesis, which is ugly.  Use this escape
     sequence whenever an oblique glyph is immediately followed by an
     upright glyph without any intervening space.

 -- Escape sequence: \,
     Apply a "left italic correction": modify the spacing of the
     following glyph so that the distance between it and the preceding
     glyph is correct if the latter is of upright shape.  For example,
     if a roman left parenthesis is immediately followed by an
     italic 'f', then in many fonts the bottom left portion of the 'f'
     overlaps the bottom of the left parenthesis, which is ugly.  Use
     this escape sequence whenever an upright glyph is followed
     immediately by an oblique glyph without any intervening space.

5.19.10 Dummy Characters
------------------------

As discussed in *note Requests and Macros::, the first character on an
input line is treated specially.  Further, formatting a glyph has many
consequences on formatter state (*note Environments::).  Occasionally,
we want to escape this context or embrace some of those consequences
without actually rendering a glyph to the output.

 -- Escape sequence: \&
     Interpolate a dummy character, which is constitutive of output but
     invisible.(1)  (*note Dummy Characters-Footnote-1::) Its presence
     alters the interpretation context of a subsequent input character,
     and enjoys several applications.

        * Prevent insertion of extra space after an end-of-sentence
          character.

               Test.
               Test.
                   => Test.  Test.
               Test.\&
               Test.
                   => Test. Test.

        * Prevent recognition of a control character.

               .Test
                   error-> warning: macro 'Test' not defined
               \&.Test
                   => .Test

        * Prevent kerning between two glyphs.

        * Translate a character to "nothing".

               .tr JIjiK\&k\&UVuv
               Post universitum, alea jacta est, OK?
                   => Post vniversitvm, alea iacta est, O?

     The dummy character escape sequence sees use in macro definitions
     as a means of ensuring that arguments are treated as text even if
     they begin with spaces or control characters.

          .de HD \" typeset a simple bold heading
          .  sp
          .  ft B
          \&\\$1 \" exercise: remove the \&
          .  ft
          .  sp
          ..
          .HD .\|.\|.\|surprised?

   One way to think about the dummy character is to imagine placing the
symbol '&' in the input at a certain location; if doing so has all the
side effects on formatting that you desire except for sticking an ugly
ampersand in the midst of your text, the dummy character is what you
want in its place.

 -- Escape sequence: \)
     Interpolate a transparent dummy character--one that is transparent
     to end-of-sentence detection.  It behaves as '\&', except that '\&'
     is treated as letters and numerals normally are after '.', '?' and
     '!'; '\&' cancels end-of-sentence detection, and '\)' does not.

          .de Suffix-&
          .  nop \&\\$1
          ..
          .
          .de Suffix-)
          .  nop \)\\$1
          ..
          .
          Here's a sentence.\c
          .Suffix-& '
          Another one.\c
          .Suffix-) '
          And a third.
              => Here's a sentence.' Another one.'  And a third.

   (1) Opinions of this escape sequence's name abound.  "Zero-width
space" is a popular misnomer: 'roff' formatters do not treat it like a
space.  Ossanna called it a "non-printing, zero-width character", but
the character causes _output_ even though it does not "print".  If no
output line is pending, the dummy character starts one.  Contrast an
empty input document with one containing only '\&'.  The former produces
no output; the latter, a blank page.

5.20 Manipulating Type Size and Vertical Spacing
================================================

These concepts were introduced in *note Page Geometry::.  The height of
a font's tallest glyph is one em, which is equal to the type size in
points.(1)  (*note Manipulating Type Size and Vertical
Spacing-Footnote-1::) A vertical spacing of less than 120% of the type
size can make a document hard to read.  Larger proportions can be useful
to spread the text for annotations or proofreader's marks.  By default,
GNU 'troff' uses 10 point type on 12 point spacing.  Typographers call
the difference between type size and vertical spacing "leading".(2)
(*note Manipulating Type Size and Vertical Spacing-Footnote-2::)

   (1) In text fonts, the tallest glyphs are typically parentheses.
Unfortunately, in many cases the actual dimensions of the glyphs in a
font do not closely match its declared type size!  For example, in the
standard PostScript font families, 10-point Times sets better with
9-point Helvetica and 11-point Courier than if all three were used at
10 points.

   (2) Rhyme with "sledding"; mechanical typography used lead metal
(Latin _plumbum_).

5.20.1 Changing the Type Size
-----------------------------

 -- Request: .ps [size]
 -- Request: .ps +size
 -- Request: .ps -size
 -- Escape sequence: \ssize
 -- Register: \n[.s]
     Use the 'ps' request or the '\s' escape sequence to change
     (increase, decrease) the type size (in scaled points).  Specify
     SIZE as either an absolute type size, or as a relative change from
     the current size.  'ps' with no argument restores the previous
     size.  The 'ps' request's default scaling unit is 'z'.  The
     requested size is rounded to the nearest valid size (with ties
     rounding down) within the limits supported by the device.  If the
     requested size is non-positive, it is treated as 1u.

     Type size alteration is incorrectly documented in the AT&T 'troff'
     manual, which claims "if [the requested size] is invalid, the next
     larger valid size will result, with a maximum of 36".(1)  (*note
     Changing the Type Size-Footnote-1::)

     The read-only string-valued register '.s' interpolates the type
     size in points as a decimal fraction; it is associated with the
     environment (*note Environments::).  To obtain the type size in
     scaled points, interpolate the '.ps' register instead (*note Using
     Fractional Type Sizes::).

     The '\s' escape sequence supports a variety of syntax forms.

     '\sN'
          Set the type size to N points.  N must be a single digit.  If
          N is 0, restore the previous size.

     '\s+N'
     '\s-N'
          Increase or decrease the type size by N points.  N must be
          exactly one digit.

     '\s(NN'
          Set the type size to NN points.  NN must be exactly two
          digits.

     '\s+(NN'
     '\s-(NN'
     '\s(+NN'
     '\s(-NN'
          Alter the type size in points by the two-digit value NN.

     *Note Using Fractional Type Sizes::, for further syntactical forms
     of the '\s' escape sequence that additionally accept decimal
     fractions.

          snap, snap,
          .ps +2
          grin, grin,
          .ps +2
          wink, wink, \s+2nudge, nudge,\s+8 say no more!
          .ps 10

   The '\s' escape sequence affects the environment immediately and
doesn't produce an input token.  Consequently, it can be used in
requests like 'mc', which expects a single character as an argument, to
change the type size on the fly.

     .mc \s[20]x\s[0]

 -- Request: .sizes s1 s2 ... sn [0]
     The 'DESC' file specifies which type sizes are allowed by the
     output device; see *note DESC File Format::.  Use the 'sizes'
     request to change this set of permissible sizes.  Arguments are in
     scaled points; see *note Using Fractional Type Sizes::.  Each can
     be a single type size (such as '12000'), or a range of sizes (such
     as '4000-72000').  You can optionally end the list with a '0'.

   (1) The claim appears to have been true of Ossanna 'troff' for the
C/A/T device; Kernighan made device-independent 'troff' more flexible.

5.20.2 Changing the Vertical Spacing
------------------------------------

 -- Request: .vs [space]
 -- Request: .vs +space
 -- Request: .vs -space
 -- Register: \n[.v]
     Set the vertical spacing to, or alter it by, SPACE.  The default
     scaling unit is 'p'.  If 'vs' is called without an argument, the
     vertical spacing is reset to the previous value before the last
     call to 'vs'.  GNU 'troff' emits a warning in category 'range' if
     SPACE is negative; the vertical spacing is then set to the smallest
     possible positive value, the vertical motion quantum (as found in
     the '.V' register).

     '.vs 0' isn't saved in a diversion since it doesn't result in a
     vertical motion.  You must explicitly issue this request before
     interpolating the diversion.

     The read-only register '.v' contains the vertical spacing; it is
     associated with the environment (*note Environments::).

When a break occurs, GNU 'troff' performs the following procedure.

   * Move the drawing position vertically by the "extra pre-vertical
     line space", the minimum of all negative '\x' escape sequence
     arguments in the pending output line.

   * Move the drawing position vertically by the vertical line spacing.

   * Write out the pending output line.

   * Move the drawing position vertically by the "extra post-vertical
     line space", the maximum of all positive '\x' escape sequence
     arguments in the line that has just been output.

   * Move the drawing position vertically by the "post-vertical line
     spacing" (see below).

   Prefer 'vs' or 'pvs' over 'ls' to produce double-spaced documents.
'vs' and 'pvs' have finer granularity than 'ls'; moreover, some
preprocessors assume single spacing.  *Note Manipulating Spacing::,
regarding the '\x' escape sequence and the 'ls' request.

 -- Request: .pvs [space]
 -- Request: .pvs +space
 -- Request: .pvs -space
 -- Register: \n[.pvs]
     Set the post-vertical spacing to, or alter it by, SPACE.  The
     default scaling unit is 'p'.  If 'pvs' is called without an
     argument, the post-vertical spacing is reset to the previous value
     before the last call to 'pvs'.  GNU 'troff' emits a warning in
     category 'range' if SPACE is negative; the post-vertical spacing is
     then set to zero.

     The read-only register '.pvs' contains the post-vertical spacing;
     it is associated with the environment (*note Environments::).

5.20.3 Using Fractional Type Sizes
----------------------------------

AT&T 'troff' interpreted all type size measurements in points.  Combined
with integer arithmetic, this design choice made it impossible to
support, for instance, ten and a half-point type.  In GNU 'troff', an
output device can select a scaling factor that subdivides a point into
"scaled points".  A type size expressed in scaled points can thus
represent a non-integral type size.

   A "scaled point" is equal to 1/SIZESCALE points, where SIZESCALE is
specified in the device description file 'DESC', and defaults to 1.(1)
(*note Using Fractional Type Sizes-Footnote-1::) Requests and escape
sequences in GNU 'troff' interpret arguments that represent a type size
in scaled points, which the formatter multiplies by SIZESCALE and
converts to an integer.  Arguments treated in this way comprise those to
the escape sequences '\H' and '\s', to the request 'ps', the third
argument to the 'cs' request, and the second and fourth arguments to the
'tkf' request.  Scaled points may be specified explicitly with the 'z'
scaling unit.

   For example, if SIZESCALE is 1000, then a scaled point is one
thousandth of a point.  The request '.ps 10.5' is synonymous with '.ps
10.5z' and sets the type size to 10,500 scaled points, or 10.5 points.
Consequently, in GNU 'troff', the register '.s' can interpolate a
non-integral type size.

 -- Register: \n[.ps]
     This read-only register interpolates the type size in scaled
     points; it is associated with the environment (*note
     Environments::).

   It makes no sense to use the 'z' scaling unit in a numeric expression
whose default scaling unit is neither 'u' nor 'z', so GNU 'troff'
disallows this.  Similarly, it is nonsensical to use a scaling unit
other than 'z' or 'u' in a numeric expression whose default scaling unit
is 'z', and so GNU 'troff' disallows this as well.

   Another GNU 'troff' scaling unit, 's', multiplies by the number of
basic units in a scaled point.  Thus, '\n[.ps]s' is equal to '1m' by
definition.  Do not confuse the 's' and 'z' scaling units.

 -- Register: \n[.psr]
 -- Register: \n[.sr]
     Output devices may be limited in the type sizes they can employ.
     The '.s' and '.ps' registers represent the type size selected by
     the output driver as it understands a device's capability.  The
     last _requested_ type size is interpolated in scaled points by the
     read-only register '.psr' and in points as a decimal fraction by
     the read-only string-valued register '.sr'.  Both are associated
     with the environment (*note Environments::).

     For example, if a type size of 10.95 points is requested, and the
     nearest size permitted by a 'sizes' request (or by the 'sizes' or
     'sizescale' directives in the device's 'DESC' file) is 11 points,
     the output driver uses the latter value.

   The '\s' escape sequence offers the following syntax forms that work
with fractional type sizes and accept scaling units.  You may of course
give them integral arguments.  The delimited forms need not use the
neutral apostrophe; see *note Delimiters::.

'\s[N]'
'\s'N''
     Set the type size to N scaled points; N is a numeric expression
     with a default scaling unit of 'z'.

'\s[+N]'
'\s[-N]'
'\s+[N]'
'\s-[N]'
'\s'+N''
'\s'-N''
'\s+'N''
'\s-'N''
     Increase or decrease the type size by N scaled points; N is a
     numeric expression (which may start with a minus sign) with a
     default scaling unit of 'z'.

   (1) *Note Device and Font Description Files::.

5.21 Colors
===========

GNU 'troff' supports color output with a variety of color spaces and up
to 16 bits per channel.  Some devices, particularly terminals, may be
more limited.  When color support is enabled, two colors are current at
any given time: the "stroke color", with which glyphs, rules (lines),
and geometric objects like circles and polygons are drawn, and the "fill
color", which can be used to paint the interior of a closed geometric
figure.

 -- Request: .color [n]
 -- Register: \n[.color]
     If N is missing or non-zero, enable the output of color-related
     device-independent output commands (this is the default);
     otherwise, disable them.  This request sets a global flag; it does
     not produce an input token (*note Gtroff Internals::).

     The read-only register '.color' is 1 if colors are enabled,
     0 otherwise.

     Color can also be disabled with the '-c' command-line option.

 -- Request: .defcolor ident scheme color-component ...
     Define a color named IDENT.  SCHEME selects a color space and
     determines the quantity of required COLOR-COMPONENTs; it must be
     one of 'rgb' (three components), 'cmy' (three), 'cmyk' (four), or
     'gray' (one).  'grey' is accepted as a synonym of 'gray'.  The
     color components can be encoded as a single hexadecimal value
     starting with '#' or '##'.  The former indicates that each
     component is in the range 0-255 (0-FF), the latter the range
     0-65,535 (0-FFFF).

          .defcolor half gray #7f
          .defcolor pink rgb #FFC0CB
          .defcolor magenta rgb  ##ffff0000ffff

     Alternatively, each color component can be specified as a decimal
     fraction in the range 0-1, interpreted using a default scaling unit
     of 'f', which multiplies its value by 65,536 (but clamps it at
     65,535).

          .defcolor gray50 rgb 0.5 0.5 0.5
          .defcolor darkgreen rgb 0.1f 0.5f 0.2f

   Each output device has a color named 'default', which cannot be
redefined.  A device's default stroke and fill colors are not
necessarily the same.  For the 'dvi', 'html', 'pdf', 'ps', and 'xhtml'
output devices, GNU 'troff' automatically loads a macro file defining
many color names at startup.  By the same mechanism, the devices
supported by 'grotty' recognize the eight standard ISO 6429/EMCA-48
color names.(1)  (*note Colors-Footnote-1::)

 -- Request: .gcolor [color]
 -- Escape sequence: \mc
 -- Escape sequence: \m(co
 -- Escape sequence: \m[color]
 -- Register: \n[.m]
     Set the stroke color to COLOR.

          .gcolor red
          The next words
          .gcolor
          \m[red]are in red\m[]
          and these words are in the previous color.

     The escape sequence '\m[]' restores the previous stroke color, as
     does a 'gcolor' request without an argument.

     The name of the current stroke color is available in the read-only
     string-valued register '.m'; it is associated with the environment
     (*note Environments::).  It interpolates nothing when the stroke
     color is the default.

     '\m' doesn't produce an input token in GNU 'troff' (*note Gtroff
     Internals::).  It therefore can be used in requests like 'mc'
     (which expects a single character as an argument) to change the
     color on the fly:

          .mc \m[red]x\m[]

 -- Request: .fcolor [color]
 -- Escape sequence: \Mc
 -- Escape sequence: \M(co
 -- Escape sequence: \M[color]
 -- Register: \n[.M]
     Set the fill color for objects drawn with '\D'...'' escape
     sequences.  The escape sequence '\M[]' restores the previous fill
     color, as does an 'fcolor' request without an argument.

     The name of the current fill color is available in the read-only
     string-valued register '.M'; it is associated with the environment
     (*note Environments::).  It interpolates nothing when the fill
     color is the default.  '\M' doesn't produce an input token in GNU
     'troff'.

     Create an ellipse with a red interior as follows.

          \M[red]\h'0.5i'\D'E 2i 1i'\M[]

   (1) also known vulgarly as "ANSI colors"

5.22 Strings
============

GNU 'troff' supports strings primarily for user convenience.
Conventionally, if one would define a macro only to interpolate a small
amount of text, without invoking requests or calling any other macros,
one defines a string instead.  Only one string is predefined by the
language.

 -- String: \*[.T]
     Contains the name of the output device (for example, 'utf8' or
     'pdf').

   The 'ds' request creates a string with a specified name and contents
and the '\*' escape sequence dereferences its name, interpolating its
contents.  If the string named by the '\*' escape sequence does not
exist, it is defined as empty, nothing is interpolated, and a warning in
category 'mac' is emitted.  *Note Warnings::, for information about the
enablement and suppression of warnings.

 -- Request: .ds name [contents]
 -- Request: .ds1 name [contents]
 -- Escape sequence: \*n
 -- Escape sequence: \*(nm
 -- Escape sequence: \*[name [arg1 arg2 ...]]
     Define a string called NAME with contents CONTENTS.  If NAME
     already exists as an alias, the target of the alias is redefined;
     see 'als' and 'rm' below.  If 'ds' is called with only one
     argument, NAME is defined as an empty string.  Otherwise, GNU
     'troff' stores CONTENTS in copy mode.(1)  (*note
     Strings-Footnote-1::)

     The '\*' escape sequence interpolates a previously defined string
     variable NAME (one-character name N, two-character name NM).  The
     bracketed interpolation form accepts arguments that are handled as
     macro arguments are; recall *note Calling Macros::.  In contrast to
     macro calls, however, if a closing bracket ']' occurs in a string
     argument, that argument must be enclosed in double quotes.  '\*' is
     interpreted even in copy mode.  When defining strings, argument
     interpolations must be escaped if they are to reference parameters
     from the calling context; *Note Parameters::.

          .ds cite (\\$1, \\$2)
          Gray codes are explored in \*[cite Morgan 1998].
              => Gray codes are explored in (Morgan, 1998).

     *Caution:* Unlike other requests, the second argument to the 'ds'
     request consumes the remainder of the input line, including
     trailing spaces.  This means that comments on a line with such a
     request can introduce unwanted space into a string when they are
     set off from the material they annotate, as is conventional.

          .ds H2O H\v'+.3m'\s'-2'2\v'-.3m'\s0O \" water

     Instead, place the comment on another line or put the comment
     escape sequence immediately adjacent to the last character of the
     string.

          .ds H2O H\v'+.3m'\s'-2'2\v'-.3m'\s0O\" water

     Ending string definitions (and appendments) with a comment, even an
     empty one, prevents unwanted space from creeping into them during
     source document maintenance.

          .ds author Alice Pleasance Liddell\"
          .ds empty \" might be appended to later with .as

     An initial neutral double quote '"' in CONTENTS is stripped to
     allow embedding of leading spaces.  Any other '"' is interpreted
     literally, but it is wise to use the special character escape
     sequence '\[dq]' instead if the string might be interpolated as
     part of a macro argument; see *note Calling Macros::.

          .ds salutation "         Yours in a white wine sauce,\"
          .ds c-var-defn "  char mydate[]=\[dq]2020-07-29\[dq];\"

     Strings are not limited to a single input line of text.  '\<RET>'
     works just as it does elsewhere.  The resulting string is stored
     _without_ the newlines.  Care is therefore required when
     interpolating strings while filling is disabled.

          .ds foo This string contains \
          text on multiple lines \
          of input.

     It is not possible to embed a newline in a string that will be
     interpreted as such when the string is interpolated.  To achieve
     that effect, use '\*' to interpolate a macro instead; see *note
     Punning Names::.

     Because strings are similar to macros, they too can be defined so
     as to suppress AT&T 'troff' compatibility mode when used; see *note
     Writing Macros:: and *note Compatibility Mode::.  The 'ds1' request
     defines a string such that compatibility mode is off when the
     string is later interpolated.  To be more precise, a "compatibility
     save" input token is inserted at the beginning of the string, and a
     "compatibility restore" input token at the end.

          .nr xxx 12345
          .ds aa The value of xxx is \\n[xxx].
          .ds1 bb The value of xxx is \\n[xxx].
          .
          .cp 1
          .
          \*(aa
              error-> warning: register '[' not defined
              => The value of xxx is 0xxx].
          \*(bb
              => The value of xxx is 12345.

 -- Request: .as name [contents]
 -- Request: .as1 name [contents]
     The 'as' request is similar to 'ds' but appends CONTENTS to the
     string stored as NAME instead of redefining it.  If NAME doesn't
     exist yet, it is created.  If 'as' is called with only one
     argument, no operation is performed (beyond dereferencing the
     string).

          .as salutation " with shallots, onions and garlic,\"

     The 'as1' request is similar to 'as', but compatibility mode is
     switched off when the appended portion of the string is later
     interpolated.  To be more precise, a "compatibility save" input
     token is inserted at the beginning of the appended string, and a
     "compatibility restore" input token at the end.

   Several requests exist to perform rudimentary string operations.
Strings can be queried ('length') and modified ('chop', 'substring',
'stringup', 'stringdown'), and their names can be manipulated through
renaming, removal, and aliasing ('rn', 'rm', 'als').

 -- Request: .length reg anything
     Compute the number of characters of ANYTHING and store the count in
     the register REG.  If REG doesn't exist, it is created.  ANYTHING
     is read in copy mode.

          .ds xxx abcd\h'3i'efgh
          .length yyy \*[xxx]
          \n[yyy]
              => 14

 -- Request: .chop object
     Remove the last character from the macro, string, or diversion
     named OBJECT.  This is useful for removing the newline from the end
     of a diversion that is to be interpolated as a string.  This
     request can be used repeatedly on the same OBJECT; see *note Gtroff
     Internals::, for details on nodes inserted additionally by GNU
     'troff'.

 -- Request: .substring str start [end]
     Replace the string named STR with its substring bounded by the
     indices START and END, inclusively.  The first character in the
     string has index 0.  If END is omitted, it is implicitly set to the
     largest valid value (the string length minus one).  Negative
     indices count backward from the end of the string: the last
     character has index -1, the character before the last has index -2,
     and so on.

          .ds xxx abcdefgh
          .substring xxx 1 -4
          \*[xxx]
              => bcde
          .substring xxx 2
          \*[xxx]
              => de

 -- Request: .stringdown str
 -- Request: .stringup str
     Alter the string named STR by replacing each of its bytes with its
     lowercase ('stringdown') or uppercase ('stringup') version (if one
     exists).  Special characters in the string will often transform in
     the expected way due to the regular naming convention for accented
     characters.  When they do not, use substrings and/or catenation.

          .ds resume R\['e]sum\['e]
          \*[resume]
          .stringdown resume
          \*[resume]
          .stringup resume
          \*[resume]
              => R�sum� r�sum� R�SUM�

   (In practice, we would end the 'ds' request with a comment escape
'\"' to prevent space from creeping into the definition during source
document maintenance.)

 -- Request: .rn old new
     Rename the request, macro, diversion, or string OLD to NEW.

 -- Request: .rm name
     Remove the request, macro, diversion, or string NAME.  GNU 'troff'
     treats subsequent invocations as if the name had never been
     defined.

 -- Request: .als new old
     Create an alias NEW for the existing request, string, macro, or
     diversion object named OLD, causing the names to refer to the same
     stored object.  If OLD is undefined, a warning in category 'mac' is
     produced, and the request is ignored.  *Note Warnings::, for
     information about the enablement and suppression of warnings.

     To understand how the 'als' request works, consider two different
     storage pools: one for objects (macros, strings, etc.), and another
     for names.  As soon as an object is defined, GNU 'troff' adds it to
     the object pool, adds its name to the name pool, and creates a link
     between them.  When 'als' creates an alias, it adds a new name to
     the name pool that gets linked to the same object as the old name.

     Now consider this example.

          .de foo
          ..
          .
          .als bar foo
          .
          .de bar
          .  foo
          ..
          .
          .bar
              error-> input stack limit exceeded (probable infinite
              error-> loop)

     In the above, 'bar' remains an _alias_--another name for--the
     object referred to by 'foo', which the second 'de' request
     replaces.  Alternatively, imagine that the 'de' request
     _dereferences_ its argument before replacing it.  Either way, the
     result of calling 'bar' is a recursive loop that finally leads to
     an error.  *Note Writing Macros::.

     To remove an alias, call 'rm' on its name.  The object itself is
     not destroyed until it has no more names.

     When a request, macro, string, or diversion is aliased,
     redefinitions and appendments "write through" alias names.  To
     replace an alias with a separately defined object, you must use the
     'rm' request on its name first.

   (1) *Note Copy Mode::.

5.23 Conditionals and Loops
===========================

'groff' has 'if' and 'while' control structures like other languages.
However, the syntax for grouping multiple input lines in the branches or
bodies of these structures is unusual.

5.23.1 Operators in Conditionals
--------------------------------

In 'if', 'ie', and 'while' requests, in addition to the numeric
expressions described in *note Numeric Expressions::, several Boolean
operators are available; the members of this expanded class are termed
"conditional expressions".

'c GLYPH'
     True if GLYPH is available, where GLYPH is an ordinary character, a
     special character '\(XX' or '\[XXX]', '\N'XXX'', or has been
     defined by any of the 'char', 'fchar', 'fschar', or 'schar'
     requests.

'd NAME'
     True if a string, macro, diversion, or request called NAME exists.

'e'
     True if the current page is even-numbered.

'F FONT'
     True if FONT exists.  FONT is handled as if it were opened with the
     'ft' request (that is, font translation and styles are applied),
     without actually mounting it.

'm COLOR'
     True if COLOR is defined.

'n'
     True if the document is being processed in 'nroff' mode.  *Note
     troff and nroff Modes::.

'o'
     True if the current page is odd-numbered.

'r REGISTER'
     True if REGISTER exists.

'S STYLE'
     True if STYLE is available for the current font family.  Font
     translation is applied.

't'
     True if the document is being processed in 'troff' mode.  *Note
     troff and nroff Modes::.

'v'
     Always false.  This condition is recognized only for compatibility
     with certain other 'troff' implementations.(1)  (*note Operators in
     Conditionals-Footnote-1::)

   If the first argument to an 'if', 'ie', or 'while' request begins
with a non-alphanumeric character apart from '!' (see below); it
performs an output comparison test.  (2)  (*note Operators in
Conditionals-Footnote-2::)

''XXX'YYY''
     True if formatting the comparands XXX and YYY produces the same
     output commands.  The delimiter need not be a neutral apostrophe:
     the output comparison operator accepts the same delimiters as most
     escape sequences; see *note Delimiters::.  This "output comparison
     operator" formats XXX and YYY in separate environments; after the
     comparison, the resulting data are discarded.

          .ie "|"\fR|\fP" true
          .el false
              => true

     The resulting glyph properties, including font family, style, size,
     and slant, must match, but not necessarily the requests and/or
     escape sequences used to obtain them.  In the previous example, '|'
     and '\fR|\fP' result in '|' glyphs in the same typefaces at the
     same positions, so the comparands are equal.  If '.ft I' had been
     added before the '.ie', they would differ: the first '|' would
     produce an italic '|', not a roman one.  Motions must match in
     orientation and magnitude to within the applicable horizontal and
     vertical motion quanta of the device, after rounding.  '.if
     "\u\d"\v'0'"' is false even though both comparands result in zero
     net motion, because motions are not interpreted or optimized but
     sent as-is to the output.(3)  (*note Operators in
     Conditionals-Footnote-3::) On the other hand, '.if "\d"\v'0.5m'"'
     is true, because '\d' is defined as a downward motion of one-half
     em.(4)  (*note Operators in Conditionals-Footnote-4::)

     Surround the comparands with '\?' to avoid formatting them; this
     causes them to be compared character by character, as with string
     comparisons in other programming languages.

          .ie "\?|\?"\?\fR|\fP\?" true
          .el false
              => false

     Since comparands protected with '\?' are read in copy mode (*note
     Copy Mode::), they need not even be valid 'groff' syntax.  The
     escape character is still lexically recognized, however, and
     consumes the next character.

          .ds a \[
          .ds b \[
          .if '\?\*a\?'\?\*b\?' a and b equivalent
          .if '\?\\?'\?\\?' backslashes equivalent
              => a and b equivalent

   The above operators can't be combined with most others, but a leading
'!', not followed immediately by spaces or tabs, complements an
expression.

     .nr x 1
     .ie !r x register x is not defined
     .el      register x is defined
         => register x is defined

   Spaces and tabs are optional immediately after the 'c', 'd', 'F',
'm', 'r', and 'S' operators, but right after '!', they end the predicate
and the conditional evaluates true.(5)  (*note Operators in
Conditionals-Footnote-5::)

     .nr x 1
     .ie ! r x register x is not defined
     .el       register x is defined
         => r x register x is not defined

The unexpected 'r x' in the output is a clue that our conditional was
not interpreted as we planned, but matters may not always be so obvious.

   (1) This refers to 'vtroff', a translator that would convert the
C/A/T output from early-vintage AT&T 'troff' to a form suitable for
Versatec and Benson-Varian plotters.

   (2) Strictly, letters not otherwise recognized _are_ treated as
output comparison delimiters.  For portability, it is wise to avoid
using letters not in the list above; for example, Plan 9 'troff' uses
'h' to test a mode it calls 'htmlroff', and GNU 'troff' may provide
additional operators in the future.

   (3) Because formatting of the comparands takes place in a dummy
environment, vertical motions within them cannot spring traps.

   (4) All of this is to say that the lists of output nodes created by
formatting XXX and YYY must be identical.  *Note Gtroff Internals::.

   (5) This bizarre behavior maintains compatibility with AT&T 'troff'.

5.23.2 if-then
--------------

 -- Request: .if cond-expr anything
     Evaluate the conditional expression COND-EXPR, and if it evaluates
     true (or to a positive value), interpret the remainder of the line
     ANYTHING as if it were an input line.  Recall from *note Invoking
     Requests:: that any quantity of spaces between arguments to
     requests serves only to separate them; leading spaces in ANYTHING
     are thus not seen.  ANYTHING effectively _cannot_ be omitted; if
     COND-EXPR is true and ANYTHING is empty, the newline at the end of
     the control line is interpreted as a blank input line (and
     therefore a blank text line).

          super\c
          tanker
          .nr force-word-break 1
          super\c
          .if ((\n[force-word-break] = 1) & \n[.int])
          tanker
              => supertanker super tanker

 -- Request: .nop anything
     Interpret ANYTHING as if it were an input line.  This is similar to
     '.if 1'.  'nop' is not really "no operation"; its argument _is_
     processed--unconditionally.  It can be used to cause text lines to
     share indentation with surrounding control lines.

          .als real-MAC MAC
          .de wrapped-MAC
          .  tm MAC: called with arguments \\$@
          .  nop \\*[real-MAC]\\
          ..
          .als MAC wrapped-MAC
          \# Later...
          .als MAC real-MAC

     In the above, we've used aliasing, 'nop', and the interpolation of
     a macro as a string to interpose a wrapper around the macro 'MAC'
     (perhaps to debug it).

5.23.3 if-else
--------------

 -- Request: .ie cond-expr anything
 -- Request: .el anything
     Use the 'ie' and 'el' requests to write an if-then-else.  The first
     request is the "if" part and the latter is the "else" part.
     Unusually among programming languages, any number of
     non-conditional requests may be interposed between the 'ie' branch
     and the 'el' branch.

          .nr a 0
          .ie \na a is non-zero.
          .nr a +1
          .el a was not positive but is now \na.
              => a was not positive but is now 1.

     Another way in which 'el' is an ordinary request is that it does
     not lexically "bind" more tightly to its 'ie' counterpart than it
     does to any other request.  This fact can surprise C programmers.

          .nr a 1
          .nr z 0
          .ie \nz \
          .  ie \na a is true
          .  el     a is false
          .el z is false
              error-> warning: unbalanced 'el' request
              => a is false

     To conveniently nest conditionals, keep reading.

5.23.4 Conditional Blocks
-------------------------

It is frequently desirable for a control structure to govern more than
one request, macro call, text line, or a combination of the foregoing.
The opening and closing brace escape sequences '\{' and '\}' define such
groups.  These "conditional blocks" can furthermore be nested.

 -- Escape sequence: \{
 -- Escape sequence: \}
     '\{' begins a conditional block; it must appear (after optional
     spaces and tabs) immediately subsequent to the conditional
     expression of an 'if', 'ie', or 'while' request,(1) (*note
     Conditional Blocks-Footnote-1::) or as the argument to an 'el'
     request.

     '\}' ends a condition block and should appear on a line with other
     occurrences of itself as necessary to match '\{' sequences.  It can
     be preceded by a control character, spaces, and tabs.  Input after
     any quantity of '\}' sequences on the same line is processed only
     if all of the preceding conditions to which they correspond are
     true.  Furthermore, a '\}' closing the body of a 'while' request
     must be the last such escape sequence on an input line.

     Brace escape sequences outside of control structures have no
     meaning and produce no output.

     *Caution:* Input lines using '\{' often end with '\RET', especially
     in macros that consist primarily of control lines.  Forgetting to
     use '\RET' on an input line after '\{' is a common source of error.

   We might write the following in a page header macro.  If we delete
'\RET', the header will carry an unwanted extra empty line (except on
page 1).

     .if (\\n[%] != 1) \{\
     .  ie ((\\n[%] % 2) = 0) .tl \\*[even-numbered-page-title]
     .  el                    .tl \\*[odd-numbered-page-title]
     .\}

   Let us take a closer look at how conditional blocks nest.

     A
     .if 0 \{ B
     C
     D
     \}E
     F
         => A F

     N
     .if 1 \{ O
     .  if 0 \{ P
     Q
     R\} S\} T
     U
         => N O U

   The above behavior may challenge the intuition; it was implemented to
retain compatibility with AT&T 'troff'.  For clarity, it is idiomatic to
end input lines with '\{' (followed by '\<RET>' if appropriate), and to
precede '\}' on an input line with nothing more than a control
character, spaces, tabs, and other instances of itself.

   We can use 'ie', 'el', and conditional blocks to simulate the
multi-way "switch" or "case" control structures of other languages.  The
following example is adapted from the 'groff' 'man' package.
Indentation is used to clarify the logic.

     .\" Simulate switch/case in roff.
     .      ie '\\$2'1' .ds title General Commands\"
     .el \{.ie '\\$2'2' .ds title System Calls\"
     .el \{.ie '\\$2'3' .ds title Library Functions\"
     .el \{.ie '\\$2'4' .ds title Kernel Interfaces\"
     .el \{.ie '\\$2'5' .ds title File Formats\"
     .el \{.ie '\\$2'6' .ds title Games\"
     .el \{.ie '\\$2'7' .ds title Miscellaneous Information\"
     .el \{.ie '\\$2'8' .ds title System Management\"
     .el \{.ie '\\$2'9' .ds title Kernel Development\"
     .el                .ds title \" empty
     .\}\}\}\}\}\}\}\}

   (1) *Note while::.

5.23.5 while
------------

'groff' provides a looping construct: the 'while' request.  Its syntax
matches the 'if' request.

 -- Request: .while cond-expr anything
     Evaluate the conditional expression COND-EXPR, and repeatedly
     execute ANYTHING unless and until COND-EXPR evaluates false.
     ANYTHING, which is often a conditional block, is referred to as the
     'while' request's "body".

          .nr a 0 1
          .while (\na < 9) \{\
          \n+a,
          .\}
          \n+a
              => 1, 2, 3, 4, 5, 6, 7, 8, 9, 10

     GNU 'troff' treats the body of a 'while' request similarly to that
     of a 'de' request (albeit one not read in copy mode(1) (*note
     while-Footnote-1::)), but stores it under an internal name and
     deletes it when the loop finishes.  The operation of a macro
     containing a 'while' request can slow significantly if the 'while'
     body is large.  Each time the macro is executed, the 'while' body
     is parsed and stored again.

          .de xxx
          .  nr num 10
          .  while (\\n[num] > 0) \{\
          .    \" many lines of code
          .    nr num -1
          .  \}
          ..

     An often better solution--and one that is more portable, since AT&T
     'troff' lacked the 'while' request--is to instead write a recursive
     macro.  It will be parsed only once.(2)  (*note while-Footnote-2::)

          .de yyy
          .  if (\\n[num] > 0) \{\
          .    \" many lines of code
          .    nr num -1
          .    yyy
          .  \}
          ..
          .
          .de xxx
          .  nr num 10
          .  yyy
          ..

     To prevent infinite loops, the default number of available
     recursion levels is 1,000 or somewhat less.(3)  (*note
     while-Footnote-3::) You can disable this protective measure, or
     raise the limit, by setting the 'slimit' register.  *Note
     Debugging::.

     As noted above, if a 'while' body begins with a conditional block,
     its closing brace must end an input line.

          .if 1 \{\
          .  nr a 0 1
          .  while (\n[a] < 10) \{\
          .    nop \n+[a]
          .\}\}
              error-> unbalanced brace escape sequences

 -- Request: .break
     Exit a 'while' loop.  Do not confuse this request with a
     typographical break or the 'br' request.

 -- Request: .continue
     Skip the remainder of a 'while' loop's body, immediately starting
     the next iteration.

   (1) *Note Copy Mode::.

   (2) unless you redefine it

   (3) "somewhat less" because things other than macro calls can be on
the input stack

5.24 Writing Macros
===================

A "macro" is a stored collection of text and control lines that can be
interpolated multiple times.  Use macros to define common operations.
Macros are called in the same way that requests are invoked.  While
requests exist for the purpose of creating macros, simply calling an
undefined macro, or interpolating it as a string, will cause it to be
defined as empty.  *Note Identifiers::.

 -- Request: .de name [end]
     Define a macro NAME, replacing the definition of any existing
     request, macro, string, or diversion called NAME.  If NAME already
     exists as an alias, the target of the alias is redefined; recall
     *note Strings::.  GNU 'troff' enters copy mode,(1) (*note Writing
     Macros-Footnote-1::) storing subsequent input lines as the macro
     definition.  If the optional second argument is not specified, the
     definition ends with the control line '..' (two dots).
     Alternatively, END identifies a macro whose call syntax at the
     start of a control line ends the definition of NAME; END is then
     called normally.  A macro definition must end in the same
     conditional block (if any) in which it began (*note Conditional
     Blocks::).  Spaces or tabs are permitted after the control
     character in the line containing this ending token (either '.' or
     'END'), but a tab immediately after the token prevents its
     recognition as the end of a macro definition.  The macro END can be
     called with arguments.(2)  (*note Writing Macros-Footnote-2::)

     Here is a small example macro called 'P' that causes a break and
     inserts some vertical space.  It could be used to separate
     paragraphs.

          .de P
          .  br
          .  sp .8v
          ..

     We can define one macro within another.  Attempting to nest '..'
     na�vely will end the outer definition because the inner definition
     isn't interpreted as such until the outer macro is later
     interpolated.  We can use an end macro instead.  Each level of
     nesting should use a unique end macro.

     An end macro need not be defined until it is called.  This fact
     enables a nested macro definition to begin inside one macro and end
     inside another.  Consider the following example.(3)  (*note Writing
     Macros-Footnote-3::)

          .de m1
          .  de m2 m3
          you
          ..
          .de m3
          Hello,
          Joe.
          ..
          .de m4
          do
          ..
          .m1
          know?
          .  m3
          What
          .m4
          .m2
              => Hello, Joe.  What do you know?

     A nested macro definition _can_ be terminated with '..' and nested
     macros _can_ reuse end macros, but these control lines must be
     escaped multiple times for each level of nesting.  The necessity of
     this escaping and the utility of nested macro definitions will
     become clearer when we employ macro parameters and consider the
     behavior of copy mode in detail.

   'de' defines a macro that inherits the compatibility mode enablement
status of its context (*note Implementation Differences::).  Often it is
desirable to make a macro that uses 'groff' features callable from
contexts where compatibility mode is on; for instance, when writing
extensions to a historical macro package.  To achieve this,
compatibility mode needs to be switched off while such a macro is
interpreted--without disturbing that state when it is finished.

 -- Request: .de1 name [end]
     The 'de1' request defines a macro to be interpreted with
     compatibility mode disabled.  When NAME is called, compatibility
     mode enablement status is saved; it is restored when the call
     completes.  Observe the extra backlash before the interpolation of
     register 'xxx'; we'll explore this subject in *note Copy Mode::.

          .nr xxx 12345
          .de aa
          The value of xxx is \\n[xxx].
          .  br
          ..
          .de1 bb
          The value of xxx is \\n[xxx].
          ..
          .cp 1
          .aa
              error-> warning: register '[' not defined
              => The value of xxx is 0xxx].
          .bb
              => The value of xxx is 12345.

 -- Request: .dei name [end]
 -- Request: .dei1 name [end]
     The 'dei' request defines a macro with its name and end macro
     indirected through strings.  That is, it interpolates strings named
     NAME and END before performing the definition.

     The following examples are equivalent.

          .ds xx aa
          .ds yy bb
          .dei xx yy

          .de aa bb

     The 'dei1' request bears the same relationship to 'dei' as 'de1'
     does to 'de'; it temporarily turns compatibility mode off when NAME
     is called.

 -- Request: .am name [end]
 -- Request: .am1 name [end]
 -- Request: .ami name [end]
 -- Request: .ami1 name [end]
     'am' appends subsequent input lines to macro NAME, extending its
     definition, and otherwise working as 'de' does.

     To make the previously defined 'P' macro set indented instead of
     block paragraphs, add the necessary code to the existing macro.

          .am P
          .ti +5n
          ..

     The other requests are analogous to their 'de' counterparts.  The
     'am1' request turns off compatibility mode during interpretation of
     the appendment.  The 'ami' request appends indirectly, meaning that
     strings NAME and END are interpolated with the resulting names used
     before appending.  The 'ami1' request is similar to 'ami',
     disabling compatibility mode during interpretation of the appended
     lines.

   Using 'trace.tmac', you can trace calls to 'de', 'de1', 'am', and
'am1'.  You can also use the 'backtrace' request at any point desired to
troubleshoot tricky spots (*note Debugging::).

   *Note Strings::, for the 'als', 'rm', and 'rn' requests to create an
alias of, remove, and rename a macro, respectively.

   Macro identifiers share their name space with requests, strings, and
diversions; see *note Identifiers::.  The 'am', 'as', 'da', 'de', 'di',
and 'ds' requests (together with their variants) create a new object
only if the name of the macro, diversion, or string is currently
undefined or if it is defined as a request; normally, they modify the
value of an existing object.  *Note the description of the 'als'
request: als, for pitfalls when redefining a macro that is aliased.

 -- Request: .return [anything]
     Exit a macro, immediately returning to the caller.  If called with
     an argument ANYTHING, exit twice--the current macro and the macro
     one level higher.  This is used to define a wrapper macro for
     'return' in 'trace.tmac'.

   (1) *Note Copy Mode::.

   (2) While it is possible to define and call a macro '.', you can't
use it as an end macro: during a macro definition, '..' is never handled
as calling '.', even if '.de NAME .' explicitly precedes it.

   (3) Its structure is adapted from, and isomorphic to, part of a
solution by Tadziu Hoffman to the problem of reflowing text multiple
times to find an optimal configuration for it.
<https://lists.gnu.org/archive/html/groff/2008-12/msg00006.html>

5.24.1 Parameters
-----------------

Macro calls and string interpolations optionally accept a list of
arguments; recall *note Calling Macros::.  At the time such an
interpolation takes place, these "parameters" can be examined using a
register and a variety of escape sequences starting with '\$'.  All such
escape sequences are interpreted even in copy mode, a fact we shall
motivate and explain below (*note Copy Mode::).

 -- Register: \n[.$]
     The count of parameters available to a macro or string is kept in
     this read-only register.  The 'shift' request can change its value.

   Any individual parameter can be accessed by its position in the list
of arguments to the macro call, numbered from left to right starting at
1, with one of the following escape sequences.

 -- Escape sequence: \$n
 -- Escape sequence: \$(nn
 -- Escape sequence: \$[nnn]
     Interpolate the Nth, NNth, or NNNth parameter.  The first form
     expects only a single digit (1<=N<=9)), the second two digits
     (01<=NN<=99)), and the third any positive integer NNN.  Macros and
     strings accept an unlimited number of parameters.

 -- Request: .shift [n]
     Shift the parameters N places (1 by default).  This is a "left
     shift": what was parameter I becomes parameter I-N.  The parameters
     formerly in positions 1 to N are no longer available.  Shifting by
     a non-positive amount performs no operation.  The register '.$' is
     adjusted accordingly.

   In practice, parameter interpolations are usually seen prefixed with
an extra escape character.  This is because the '\$' family of escape
sequences is interpreted even in copy mode.(1)  (*note
Parameters-Footnote-1::)

 -- Escape sequence: \$*
 -- Escape sequence: \$@
 -- Escape sequence: \$^
     In some cases it is convenient to interpolate all of the parameters
     at once (to pass them to a request, for instance).  The '\$*'
     escape concatenates the parameters, separating them with spaces.
     '\$@' is similar, concatenating the parameters, surrounding each
     with double quotes and separating them with spaces.  If not in
     compatibility mode, the interpolation depth of double quotes is
     preserved (*note Calling Macros::).  '\$^' interpolates all
     parameters as if they were arguments to the 'ds' request.

          .de foo
          . tm $1='\\$1'
          . tm $2='\\$2'
          . tm $*='\\$*'
          . tm $@='\\$@'
          . tm $^='\\$^'
          ..
          .foo " This is a "test"
              error-> $1=' This is a '
              error-> $2='test"'
              error-> $*=' This is a  test"'
              error-> $@='" This is a " "test""'
              error-> $^='" This is a "test"'

     '\$*' is useful when writing a macro that doesn't need to
     distinguish its arguments, or even to not interpret them; examples
     include macros that produce diagnostic messages by wrapping the
     'tm' or 'ab' requests.  Use '\$@' when writing a macro that may
     need to shift its parameters and/or wrap a macro or request that
     finds the count significant.  If in doubt, prefer '\$@' to '\$*'.
     An application of '\$^' is seen in 'trace.tmac', which redefines
     some requests and macros for debugging purposes.

 -- Escape sequence: \$0
     Interpolate the name by which the macro being interpreted was
     called.  The 'als' request can cause a macro to have more than one
     name.  Applying string interpolation to a macro does not change
     this name.

          .de foo
          .  tm \\$0
          ..
          .als bar foo
          .
          .de aaa
          .  foo
          ..
          .de bbb
          .  bar
          ..
          .de ccc
          \\*[foo]\\
          ..
          .de ddd
          \\*[bar]\\
          ..
          .
          .aaa
              error-> foo
          .bbb
              error-> bar
          .ccc
              error-> ccc
          .ddd
              error-> ddd

   (1) If they were not, parameter interpolations would be similar to
command-line parameters--fixed for the entire duration of a 'roff'
program's run.  The advantage of interpolating '\$' escape sequences
even in copy mode is that they can interpolate different contents from
one call to the next, like function parameters in a procedural language.
The additional escape character is the price of this power.

5.24.2 Copy Mode
----------------

When GNU 'troff' processes certain requests, most importantly those
which define or append to a macro or string, it does so in "copy mode":
it copies the characters of the definition into a dedicated storage
region, interpolating the escape sequences '\n', '\g', '\$', '\*', '\V',
and '\?' normally; interpreting '\<RET>' immediately; discarding
comments '\"' and '\#'; interpolating the current leader, escape, or tab
character with '\a', '\e', and '\t', respectively; and storing all other
escape sequences in an encoded form.

   The complement of copy mode--a 'roff' formatter's behavior when not
defining or appending to a macro, string, or diversion--where all macros
are interpolated, requests invoked, and valid escape sequences processed
immediately upon recognition, can be termed "interpretation mode".

 -- Escape sequence: \\
     The escape character, '\' by default, can escape itself.  This
     enables you to control whether a given '\n', '\g', '\$', '\*',
     '\V', or '\?' escape sequence is interpreted at the time the macro
     containing it is defined, or later when the macro is called.(1)
     (*note Copy Mode-Footnote-1::)

          .nr x 20
          .de y
          .nr x 10
          \&\nx
          \&\\nx
          ..
          .y
              => 20 10

     You can think of '\\' as a "delayed" backslash; it is the escape
     character followed by a backslash from which the escape character
     has removed its special meaning.  Consequently, '\\' is not an
     escape sequence in the usual sense.  In any escape sequence '\X'
     that GNU 'troff' does not recognize, the escape character is
     ignored and X is output.  An unrecognized escape sequence causes a
     warning in category 'escape', with two exceptions--'\\' is the
     first.

 -- Escape sequence: \.
     '\.' escapes the control character.  It is similar to '\\' in that
     it isn't a true escape sequence.  It is used to permit nested macro
     definitions to end without a named macro call to conclude them.
     Without a syntax for escaping the control character, this would not
     be possible.

          .de m1
          foo
          .
          .  de m2
          bar
          \\..
          .
          ..
          .m1
          .m2
              => foo bar

     The first backslash is consumed while the macro is read, and the
     second is interpreted when macro 'm1' is called.

   'roff' documents should not use the '\\' or '\.' character sequences
outside of copy mode; they serve only to obfuscate the input.  Use '\e'
to represent the escape character, '\[rs]' to obtain a backslash glyph,
and '\&' before '.' and ''' where GNU 'troff' expects them as control
characters if you mean to use them literally (recall *note Requests and
Macros::).

   Macro definitions can be nested to arbitrary depth.  The mechanics of
parsing the escape character have significant consequences for this
practice.

     .de M1
     \\$1
     .  de M2
     \\\\$1
     .    de M3
     \\\\\\\\$1
     \\\\..
     .    M3 hand.
     \\..
     .  M2 of
     ..
     This understeer is getting
     .M1 out
         => This understeer is getting out of hand.

   Each escape character is interpreted twice--once in copy mode, when
the macro is defined, and once in interpretation mode, when the macro is
called.  As seen above, this fact leads to exponential growth in the
quantity of escape characters required to delay interpolation of '\n',
'\g', '\$', '\*', '\V', and '\?' at each nesting level, which can be
daunting.  GNU 'troff' offers a solution.

 -- Escape sequence: \E
     '\E' represents an escape character that is not interpreted in copy
     mode.  You can use it to ease the writing of nested macro
     definitions.

          .de M1
          .  nop \E$1
          .  de M2
          .    nop \E$1
          .    de M3
          .      nop \E$1
          \\\\..
          .    M3 better.
          \\..
          .  M2 bit
          ..
          This vehicle handles
          .M1 a
              => This vehicle handles a bit better.

     Observe that because '\.' is not a true escape sequence, we can't
     use '\E' to keep '..' from ending a macro definition prematurely.
     If the multiplicity of backslashes complicates maintenance, use end
     macros.

     '\E' is also convenient to define strings containing escape
     sequences that need to work when used in copy mode (for example, as
     macro arguments), or which will be interpolated at varying macro
     nesting depths.  We might define strings to begin and end
     superscripting as follows.(2)  (*note Copy Mode-Footnote-2::)

          .ds { \v'-.9m\s'\En[.s]*7u/10u'+.7m'
          .ds } \v'-.7m\s0+.9m'

     When the 'ec' request is used to redefine the escape character,
     '\E' also makes it easier to distinguish the semantics of an escape
     character from the other meaning(s) its character might have.
     Consider the use of an unusual escape character, '-'.

          .nr a 1
          .ec -
          .de xx
          --na
          ..
          .xx
              => -na

     This result may surprise you; some people expect '1' to be output
     since register 'a' has clearly been defined with that value.  What
     has happened?  The robotic replacement of '\' with '-' has led us
     astray.  You might recognize the sequence '--' more readily with
     the default escape character as '\-', the special character escape
     sequence for the minus sign glyph.

          .nr a 1
          .ec -
          .de xx
          -Ena
          ..
          .xx
              => 1

   (1) Compare this to the '\def' and '\edef' commands in TeX.

   (2) These are lightly adapted from the 'groff' implementation of the
'ms' macros.

5.25 Page Motions
=================

*Note Manipulating Spacing::, for a discussion of the most commonly used
request for vertical motion, 'sp', which spaces downward by one vee.

 -- Request: .mk [reg]
 -- Request: .rt [dist]
     You can "mark" a location on a page for subsequent "return".  'mk'
     takes an argument, a register name in which to store the current
     page location.  If given no argument, it stores the location in an
     internal register.  This location can be used later by the 'rt' or
     the 'sp' requests (or the '\v' escape).

     The 'rt' request returns _upward_ to the location marked with the
     last 'mk' request.  If used with an argument, it returns to a
     vertical position DIST from the top of the page (no previous call
     to 'mk' is necessary in this case).  The default scaling unit is
     'v'.

     If a page break occurs between a 'mk' request and its matching 'rt'
     request, the 'rt' request is silently ignored.

     A simple implementation of a macro to set text in two columns
     follows.

          .nr column-length 1.5i
          .nr column-gap 4m
          .nr bottom-margin 1m
          .
          .de 2c
          .  br
          .  mk
          .  ll \\n[column-length]u
          .  wh -\\n[bottom-margin]u 2c-trap
          .  nr right-side 0
          ..
          .
          .de 2c-trap
          .  ie \\n[right-side] \{\
          .    nr right-side 0
          .    po -(\\n[column-length]u + \\n[column-gap]u)
          .    \" remove trap
          .    wh -\\n[bottom-margin]u
          .  \}
          .  el \{\
          .    \" switch to right side
          .    nr right-side 1
          .    po +(\\n[column-length]u + \\n[column-gap]u)
          .    rt
          .  \}
          ..

     Now let us apply our two-column macro.

          .pl 1.5i
          .ll 4i
          This is a small test that shows how the
          rt request works in combination with mk.

          .2c
          Starting here, text is typeset in two columns.
          Note that this implementation isn't robust
          and thus not suited for a real two-column
          macro.
              => This is a small test that shows how the
              => rt request works in combination with mk.
              =>
              => Starting  here,    isn't    robust
              => text is typeset    and   thus  not
              => in two columns.    suited  for   a
              => Note that  this    real two-column
              => implementation     macro.

   Several escape sequences enable fine control of movement about the
page.

 -- Escape sequence: \v'expr'
     Vertically move the drawing position.  EXPR indicates the magnitude
     of motion: positive is downward and and negative upward.  The
     default scaling unit is 'v'.  The motion is relative to the current
     drawing position unless EXPR begins with the boundary-relative
     motion operator '|'.  *Note Numeric Expressions::.

     Text processing continues at the new drawing position; usually,
     vertical motions should be in balanced pairs to avoid a confusing
     page layout.

     '\v' will not spring a vertical position trap.  This can be useful;
     for example, consider a page bottom trap macro that prints a marker
     in the margin to indicate continuation of a footnote.  *Note
     Traps::.

   A few escape sequences that produce vertical motion are unusual.
They are thought to originate early in AT&T 'nroff' history to achieve
super- and subscripting by half-line motions on line printers and
teletypewriters before the phototypesetter made more precise positioning
available.  They are reckoned in ems--not vees--to maintain continuity
with their original purpose of moving relative to the size of the type
rather than the distance between text baselines (vees).(1)  (*note Page
Motions-Footnote-1::)

 -- Escape sequence: \r
 -- Escape sequence: \u
 -- Escape sequence: \d
     Move upward 1m, upward .5m, and downward .5m, respectively.

Let us see these escape sequences in use.

     Obtain 100 cm\u3\d of \ka\d\092\h'|\nau'\r233\dU.

   In the foregoing we have paired '\u' and '\d' to typeset a
superscript, and later a full em negative ("reverse") motion to place a
superscript above a subscript.  A numeral-width horizontal motion escape
sequence aligns the proton and nucleon numbers, while '\k' marks a
horizontal position to which '\h' returns so that we could stack them.
(We shall discuss these horizontal motion escape sequences presently.)
In serious applications, we often want to alter the type size of the
-scripts and to fine-tune the vertical motions, as the 'groff' 'ms'
package does with its super- and subscripting string definitions.

 -- Escape sequence: \h'expr'
     Horizontally move the drawing position.  EXPR indicates the
     magnitude of motion: positive is rightward and negative leftward.
     The default scaling unit is 'm'.  The motion is relative to the
     current drawing position unless EXPR begins with the
     boundary-relative motion operator '|'.  *Note Numeric
     Expressions::.

   The following string definition sets the TeX logo.(2)  (*note Page
Motions-Footnote-2::)

     .ds TeX T\h'-.1667m'\v'.224m'E\v'-.224m'\h'-.125m'X\"

   There are a number of special-case escape sequences for horizontal
motion.

 -- Escape sequence: \<SP>
     Move right one word space.  (The input is a backslash followed by a
     space.)  This escape sequence can be thought of as a
     non-adjustable, unbreakable space.  Usually you want '\~' instead;
     see *note Manipulating Filling and Adjustment::.

 -- Escape sequence: \|
     Move one-sixth em to the right on typesetting output devices.  If a
     glyph named '\|' is defined in the current font, its width is used
     instead, even on terminal output devices.

 -- Escape sequence: \^
     Move one-twelfth em to the right on typesetting output devices.  If
     a glyph named '\^' is defined in the current font, its width is
     used instead, even on terminal output devices.

 -- Escape sequence: \0
     Move right by the width of a numeral in the current font.

   Horizontal motions are not discarded at the end of an output line as
word spaces are.  *Note Breaking::.

 -- Escape sequence: \w'anything'
 -- Register: \n[st]
 -- Register: \n[sb]
 -- Register: \n[rst]
 -- Register: \n[rsb]
 -- Register: \n[ct]
 -- Register: \n[ssc]
 -- Register: \n[skw]
     Interpolate the width of ANYTHING in basic units.  This escape
     sequence allows several properties of formatted output to be
     measured without writing it out.

          The length of the string 'abc' is \w'abc'u.
              => The length of the string 'abc' is 72u.

     ANYTHING is processed in a dummy environment: this means that font
     and type size changes, for example, may occur within it without
     affecting subsequent output.

     After each use, '\w' sets several registers.

     'st'
     'sb'
          The maximum vertical displacements of the text baseline above
          and below, respectively.  The sign convention is opposite that
          of relative vertical motions; that is, depth below the
          (original) baseline is negative.  These registers are
          incorrectly documented in the AT&T 'troff' manual as "the
          highest and lowest extent of [the argument to '\w'] relative
          to the baseline".

     'rst'
     'rsb'
          Like 'st' and 'sb', but taking account of the heights and
          depths of glyphs.  In other words, these registers store the
          highest and lowest vertical positions attained by ANYTHING,
          doing what AT&T 'troff' documented 'st' and 'sb' as doing.

     'ct'
          Characterizes the geometry of glyphs occurring in ANYTHING.

          0
               only short glyphs, no descenders or tall glyphs

          1
               at least one descender

          2
               at least one tall glyph

          3
               at least one each of a descender and a tall glyph

     'ssc'
          The amount of horizontal space (possibly negative) that should
          be added to the last glyph before a subscript.

     'skw'
          How far to right of the center of the last glyph in the '\w'
          argument, the center of an accent from a roman font should be
          placed over that glyph.

 -- Escape sequence: \kp
 -- Escape sequence: \k(ps
 -- Escape sequence: \k[position]
     Store the current horizontal position in the _input_ line in a
     register with the name POSITION (one-character name P,
     two-character name PS).  Use this, for example, to return to the
     beginning of a string for highlighting or other decoration.

 -- Register: \n[hp]
     The current horizontal position at the input line.

 -- Register: \n[.k]
     A read-only register containing the current horizontal output
     position (relative to the current indentation).

 -- Escape sequence: \o'abc...'
     Overstrike the glyphs of characters A, B, C, ...; the glyphs are
     centered, written, and the drawing position advanced by the widest
     of the glyphs.

 -- Escape sequence: \zc
     Format the character C with zero width; that is, without advancing
     the drawing position.  Use '\z' to overstrike glyphs aligned to
     their left edges, in contrast to '\o''s centering.

 -- Escape sequence: \Z'anything'
     Save the drawing position, format ANYTHING, then restore it.  Tabs
     and leaders in the argument are ignored with an error diagnostic.

     We might implement a strike-through macro thus.

          .de ST
          .nr width \w'\\$1'
          \Z@\v'-.25m'\l'\\n[width]u'@\\$1
          ..
          .
          This is
          .ST "a test"
          an actual emergency!

   (1) At the 'grops' defaults of 10-point type on 12-point vertical
spacing, the difference between half a vee and half an em can be subtle:
large spacings like '.vs .5i' make it obvious.

   (2) *Note Strings::, for an explanation of the trailing '\"'.

5.26 Drawing Geometric Objects
==============================

A few of the formatter's escape sequences draw lines and other geometric
objects.  Combined with each other and with page motion commands (*note
Page Motions::), a wide variety of figures is possible.  For complex
drawings, these operations can be cumbersome; the preprocessors 'gpic'
or 'ggrn' are typically used instead.

   The '\l' and '\L' escape sequences draw horizontal and vertical
sequences of glyphs, respectively.  Even the simplest of output devices
supports them.

 -- Escape sequence: \l'l'
 -- Escape sequence: \l'lc'
     Draw a horizontal line of length L from the drawing position.
     Rightward motion is positive.  Afterward, the drawing position is
     at the right end of the line.  The default scaling unit is 'm'.

     The optional second parameter C is a character with which to draw
     the line.  The default is the baseline rule special character,
     '\[ru]'.

     If C is a valid scaling unit, put '\&' after L to disambiguate the
     input.

          .de textbox
          \[br]\\$*\[br]\l'|0\[rn]'\l'|0\[ul]'
          ..

     The foregoing outputs a box rule (a vertical line), the text
     argument(s), and another box rule.  We employ the boundary-relative
     motion operator '|'.  Finally, the line-drawing escape sequences
     draw a radical extender (a form of overline) and an underline from
     the drawing position to the position coresponding to beginning of
     the _input_ line.  The drawing position returns to just after the
     right-hand box rule because the lengths of the drawn lines are
     negative, as noted above.

 -- Escape sequence: \L'l'
 -- Escape sequence: \L'lc'
     Draw a vertical line of length L from the drawing position.
     Downward motion is positive.  The default scaling unit is 'v'.  The
     default character is the box rule, '\[br]'.  As with vertical
     motion escape sequences, text processing continues where the line
     ends.  '\L' is otherwise similar to '\l'.

          $ nroff <<EOF
          This is a \L'3v'test.
          EOF
              => This is a
              =>           |
              =>           |
              =>           |test.

     When writing text, the drawing position is at the text baseline;
     recall *note Page Geometry::.

   The '\D' escape sequence provides "drawing commands" that direct the
output device to render geometrical objects rather than glyphs.
Specific devices may support only a subset, or may feature additional
ones; consult the man page for the output driver in use.  Terminal
devices in particular implement almost none.  *Note Graphics Commands::.

   Rendering starts at the drawing position; when finished, the drawing
position is left at the rightmost point of the object, even for closed
figures, except where noted.  GNU 'troff' draws stroked (outlined)
objects with the stroke color, and shades filled ones with the fill
color.  *Note Colors::.  Coordinates H and V are horizontal and vertical
motions relative to the drawing position or previous point in the
command.  The default scaling unit for horizontal measurements (and
diameters of circles) is 'm'; for vertical ones, 'v'.

   Circles, ellipses, and polygons can be drawn filled or stroked.
These are independent properties; if you want a filled, stroked figure,
you must draw the same figure twice using each drawing command.  A
filled figure is always smaller than an outlined one because the former
is drawn only within its defined area, whereas strokes have a line
thickness (set with '\D't'').

     \h'1i'\v'1i'\
     \# increase line thickness
     \Z'\D't 5p''\
     \# draw stroked (unfilled) polygon
     \Z'\D'p 3 3 -6 0''\
     \# draw filled (solid) polygon
     \Z'\D'P 3 3 -6 0''

 -- Escape sequence: \D'command argument ...'
     Drawing command escape sequence parameters begin with an ordinary
     character, COMMAND, selecting the type of object to be drawn,
     followed by ARGUMENTs whose meaning is determined by COMMAND.

     '\D'~ H1 V1 ... HN VN''
          Draw a B-spline to each point in sequence, leaving the drawing
          position at (HN, VN).

     '\D'a HC VC H V''
          Draw a circular arc centered at (HC, VC) counterclockwise from
          the drawing position to a point (H, V) relative to the center.
          (1)  (*note Drawing Geometric Objects-Footnote-1::)

     '\D'c D''
          Draw a circle of diameter D with its leftmost point at the
          drawing position.

     '\D'C D''
          As '\D'C ...'', but the circle is filled.

     '\D'e H V''
          Draw an ellipse of width H and height V with its leftmost
          point at the drawing position.

     '\D'E X Y''
          As '\D'e ...'', but the ellipse is filled.

     '\D'l DX DY''
          Draw line from the drawing position to (H, V).

          The following is a macro for drawing a box around a text
          argument; for simplicity, the box margin is a fixed at 0.2m.

               .de TEXTBOX
               .  nr @wd \w'\\$1'
               \h'.2m'\
               \h'-.2m'\v'(.2m - \\n[rsb]u)'\
               \D'l 0 -(\\n[rst]u - \\n[rsb]u + .4m)'\
               \D'l (\\n[@wd]u + .4m) 0'\
               \D'l 0 (\\n[rst]u - \\n[rsb]u + .4m)'\
               \D'l -(\\n[@wd]u + .4m) 0'\
               \h'.2m'\v'-(.2m - \\n[rsb]u)'\
               \\$1\
               \h'.2m'
               ..

          The argument is measured with the '\w' escape sequence.  Its
          width is stored in register '@wd'.  '\w' also sets the
          registers 'rst' and 'rsb'; these contain its maximum vertical
          extents of the argument.  Then, four lines are drawn to form a
          box, offset by the box margin.

     '\D'p H1 V1 ... HN VN''
          Draw polygon with vertices at drawing position and each point
          in sequence.  GNU 'troff' closes the polygon by drawing a line
          from (HN, VN) back to the initial drawing position.
          Afterward, the drawing position is left at (HN, VN).

     '\D'P DX1 DY1 DX2 DY2 ...''
          As '\D'P ...'', but the polygon is filled.

          The following macro is like the '\D'l'' example, but shades
          the box.  We draw the box before writing the text because
          colors in GNU 'troff' have no transparency; in othe opposite
          order, the filled polygon would occlude the text.

               .de TEXTBOX
               .  nr @wd \w'\\$1'
               \h'.2m'\
               \h'-.2m'\v'(.2m - \\n[rsb]u)'\
               \M[lightcyan]\
               \D'P 0 -(\\n[rst]u - \\n[rsb]u + .4m) \
                    (\\n[@wd]u + .4m) 0 \
                    0 (\\n[rst]u - \\n[rsb]u + .4m) \
                    -(\\n[@wd]u + .4m) 0'\
               \h'.2m'\v'-(.2m - \\n[rsb]u)'\
               \M[]\
               \\$1\
               \h'.2m'
               ..

     '\D't N''
          Set the stroke thickness of geometric objects to N basic
          units.  A zero N selects the minimal supported thickness.  A
          negative N selects a thickness proportional to the type size;
          this is the default.

   In a hazy penumbra between text rendering and drawing commands we
locate the bracket-building escape sequence, '\b'.  It can assemble
apparently large glyphs by vertically stacking ordinary ones.

 -- Escape sequence: \b'contents'
     Pile and center a sequence of glyphs vertically on the output line.
     "Piling" stacks glyphs corresponding to each character in CONTENTS,
     read from left to right, and placed from top to bottom.  GNU
     'troff' separates the glyphs vertically by 1m, and the pile itself
     is centered 0.5m above the text baseline.  The horizontal drawing
     position is then advanced by the width of the widest glyph in the
     pile.

     This rather inflexible positioning algorithm doesn't work with the
     'dvi' output device since its bracket pieces vary in height.
     Instead, use the 'geqn' preprocessor.

     *note Manipulating Spacing:: describes how to adjust the vertical
     spacing of the output line with the '\x' escape sequence.

     The application of '\b' that lends its name is construction of
     brackets, braces, and parentheses when typesetting mathematics.  We
     might construct a large opening (left) brace as follows.

          \b'\[lt]\[bv]\[lk]\[bv]\[lb]'

     See 'groff_char(7)' for a list of special character identifiers.

   (1) (HC, VC) is adjusted to the point nearest the perpendicular
bisector of the arc's chord.

5.27 Deferring Output
=====================

A few 'roff' language elements are generally not used in simple
documents, but arise as page layouts become more sophisticated and
demanding.  "Environments" collect formatting parameters like line
length and typeface.  A "diversion" stores formatted output for later
use.  A "trap" is a condition on the input or output, tested
automatically by the formatter, that is associated with a macro, causing
it to be called when that condition is fulfilled.

   Footnote support often exercises all three of the foregoing features.
A simple implementation might work as follows.  A pair of macros is
defined: one starts a footnote and the other ends it.  The author calls
the first macro where a footnote marker is desired.  The macro
establishes a diversion so that the footnote text is collected at the
place in the body text where its corresponding marker appears.  An
environment is created for the footnote so that it is set at a smaller
typeface.  The footnote text is formatted in the diversion using that
environment, but it does not yet appear in the output.  The document
author calls the footnote end macro, which returns to the previous
environment and ends the diversion.  Later, after much more body text in
the document, a trap, set a small distance above the page bottom, is
sprung.  The macro called by the trap draws a line across the page and
emits the stored diversion.  Thus, the footnote is rendered.

   Diversions and traps make the text formatting process non-linear.
Let us imagine a set of text lines or paragraphs labelled 'A', 'B', and
so on.  If we set up a trap that produces text 'T' (as a page footer,
say), and we also use a diversion to store the formatted text 'D', then
a document with input text in the order 'A B C D E F' might render as 'A
B C E T F'.  The diversion 'D' will never be output if we do not call
for it.

   Environments of themselves are not a source of non-linearity in
document formatting: environment switches have immediate effect.  One
could always write a macro to change as many formatting parameters as
desired with a single convenient call.  But because diversions can be
nested and macros called by traps that are sprung by other trap-called
macros, they may be called upon in varying contexts.  For example,
consider a page header that is always to be set in Helvetica.  A
document that uses Times for most of its body text, but Courier for
displayed code examples, poses a challenge if a page break occurs in the
middle of a code display; if the header trap assumes that the "previous
font" is always Times, the rest of the example will be formatted in the
wrong typeface.  One could carefully save all formatting parameters upon
entering the trap and restore them upon leaving it, but this is verbose,
error-prone, and not future-proof as the 'groff' language develops.
Environments save us considerable effort.

5.28 Traps
==========

"Traps" are locations in the output or conditions on the input that,
when reached or fulfilled, call a specified macro.  These traps can
occur at a given location on the page, at a given location in the
current diversion (together, these are known as vertical position
traps), at a blank line, at a line with leading space characters, after
a quantity of input lines, or at the end of input.  Macros called by
traps are passed no arguments.  Setting a trap is also called "planting"
one.  It is said that a trap is "sprung" if its condition is fulfilled.

5.28.1 Vertical Position Traps
------------------------------

A "vertical position trap" calls a macro when the formatter's vertical
drawing position reaches or passes, in the downward direction, a certain
location on the output page or in a diversion.  Its applications include
setting page headers and footers, body text in multiple columns, and
footnotes.

 -- Request: .vpt [flag]
 -- Register: \n[.vpt]
     Enable vertical position traps if FLAG is non-zero or absent;
     disable them otherwise.  Vertical position traps are those set by
     the 'wh' request or by 'dt' within a diversion.  The parameter that
     controls whether vertical position traps are enabled is global.
     Initially, vertical position traps are enabled.  The current value
     is stored in the '.vpt' read-only register.

     A page can't be ejected if 'vpt' is set to zero; see *note The
     Implicit Page Trap::.

5.28.1.1 Page Location Traps
............................

A "page location trap" is a vertical position trap that applies to the
page; that is, to undiverted output.  Many can be present; manage them
with the 'wh' and 'ch' requests.

 -- Request: .wh dist [name]
     Plant macro NAME as page location trap at DIST.  The default
     scaling unit is 'v'.  Non-negative values for DIST set the trap
     relative to the top of the page; negative values set the trap
     relative to the bottom of the page.  It is not possible to plant a
     trap less than one basic unit from the page bottom: a DIST of '-0'
     is interpreted as '0', the top of the page.(1)  (*note Page
     Location Traps-Footnote-1::) An existing _visible_ trap (see below)
     at DIST is removed; this is 'wh''s sole function if NAME is
     missing.

     A trap is sprung only if it is "visible", meaning that its location
     is reachable on the page(2) (*note Page Location
     Traps-Footnote-2::) and it is not hidden by another trap at the
     same location already planted there.

     A macro package might set headers and footers as follows; this
     example configures vertical margins of one inch to the body text,
     and one half-inch to the titles.  Observe the use of the no-break
     control character with 'sp' request to position our text baselines,
     and the page number character '%' used with the 'tl' request.

          .\" hdfo.roff
          .de hd                  \" page header
          '  sp .5i
          '  tl '\\*(Ti''\\*(Da'  \" title and date strings
          '  sp .5i
          ..
          .de fo                  \" page footer
          '  sp .5i
          .  tl ''%''
          .  bp
          ..
          .wh 0   hd             \" trap at top of the page
          .wh -1i fo             \" trap 1 inch from bottom

     To use these traps, copy the above (or load it from a file with the
     'so' or 'mso' requests), then set up the strings it uses.

          .so hdfo.roff
          .ds Ti Final Report\"
          .ds Da 21 May 2023\"
          .ti
          On 5 August of last year,
          this committee tasked me with the investigation of the
          CFIT (controlled flight into terrain) incident of
          .\" ...and so on...

     A trap above the top or at or below the bottom of the page can be
     made visible by either moving it into the page area or increasing
     the page length so that the trap is on the page.  Negative trap
     values always use the _current_ page length; they are not converted
     to an absolute vertical position.  We can use the 'ptr' request to
     dump our page location traps to the standard error stream (*note
     Debugging::).  Their positions are reported in basic units; an
     'nroff' device example follows.

          .pl 5i
          .wh -1i xx
          .ptr
              error-> xx      -240
          .pl 100i
          .ptr
              error-> xx      -240

     It is possible to have more than one trap at the same location
     (although only one at a time can be visible); to achieve this, the
     traps must be defined at different locations, then moved to the
     same place with the 'ch' request.  In the following example, the
     many empty lines caused by the 'bp' request are not shown in the
     output.

          .de a
          .  nop a
          ..
          .de b
          .  nop b
          ..
          .de c
          .  nop c
          ..
          .
          .wh 1i a
          .wh 2i b
          .wh 3i c
          .bp
              => a b c
          .ch b 1i
          .ch c 1i
          .bp
              => a
          .ch a 0.5i
          .bp
              => a b

 -- Register: \n[.t]
     The read-only register '.t' holds the distance to the next vertical
     position trap.  If there are no traps between the current position
     and the bottom of the page, it contains the distance to the page
     bottom.  Within a diversion, in the absence of a diversion trap,
     this distance is the largest representable integer in basic
     units--effectively infinite.

 -- Request: .ch name [dist]
     Change the location of a trap by moving macro NAME to new location
     DIST, or by unplanting it altogether if DIST is absent.  The
     default scaling unit is 'v'.  Parameters to 'ch' are specified in
     the opposite order from 'wh'.  If NAME is the earliest planted
     macro of multiple traps at the same location, (re)moving it from
     that location exposes the macro next least recently planted at the
     same place.(3)  (*note Page Location Traps-Footnote-3::)

     Changing a trap's location is useful for building up footnotes in a
     diversion to allow more space at the bottom of the page for them.

   The same macro can be installed simultaneously at multiple locations;
however, only the earliest-planted instance--that has not yet been
deleted with 'wh'--will be moved by 'ch'.  The following example (using
an 'nroff' device) illustrates this behavior.  Blank lines have been
elided from the output.

     .de T
     Trap sprung at \\n(nlu.
     .br
     ..
     .wh 1i T
     .wh 2i T
     foo
     .sp 11i
     .bp
     .ch T 4i
     bar
     .sp 11i
     .bp
     .ch T 5i
     baz
     .sp 11i
     .bp
     .wh 5i
     .ch T 6i
     qux
     .sp 11i
         => foo
         => Trap sprung at 240u.
         => Trap sprung at 480u.
         => bar
         => Trap sprung at 480u.
         => Trap sprung at 960u.
         => baz
         => Trap sprung at 480u.
         => Trap sprung at 1200u.
         => qux
         => Trap sprung at 1440u.

 -- Register: \n[.ne]
     The read-only register '.ne' contains the amount of space that was
     needed in the last 'ne' request that caused a trap to be sprung; it
     is useful in conjunction with the '.trunc' register.  *Note Page
     Control::.  Since the '.ne' register is set only by traps, it
     doesn't make sense to interpolate it outside of macros called by
     traps.

 -- Register: \n[.trunc]
     A read-only register containing the amount of vertical space
     truncated from an 'sp' request by the most recently sprung vertical
     position trap, or, if the trap was sprung by an 'ne' request, minus
     the amount of vertical motion produced by the 'ne' request.  In
     other words, at the point a trap is sprung, it represents the
     difference of what the vertical position would have been but for
     the trap, and what the vertical position actually is.  Since the
     '.trunc' register is set only by traps, it doesn't make sense to
     interpolate it outside of macros called by traps.

 -- Register: \n[.pe]
     This Boolean-valued, read-only register interpolates 1 while a page
     is being ejected, and 0 otherwise.

     In the following example, we plant the same trap at the top and the
     bottom of the page.  We also make the trap report its name and the
     vertical drawing position.

          .de T
          .tm \\$0: page \\n%, nl=\\n[nl] .pe=\\n[.pe]
          ..
          .ll 46n
          .wh 0 T
          .wh -1v T
          Those who can make you believe absurdities can make you
          commit atrocities. \[em] Voltaire
              error-> T: page 1, nl=0 .pe=0
              error-> T: page 1, nl=2600 .pe=1
              => Those who can make you believe absurdities can
              => make you commit atrocities. -- Voltaire

   When designing macros, keep in mind that diversions and traps do
normally interact.  For example, if a trap calls a header macro (while
outputting a diversion) that tries to change the font on the current
page, the effect is not visible before the diversion has completely been
printed (except for input protected with '\!' or '\?') since the data in
the diversion is already formatted.  In most cases, this is not the
expected behaviour.

   (1) *Note The Implicit Page Trap::.

   (2) A trap planted at '20i' or '-30i' will not be sprung on a page of
length '11i'.

   (3) It may help to think of each trap location as maintaining a
queue; 'wh' operates on the head of the queue, and 'ch' operates on its
tail.  Only the trap at the head of the queue is visible.

5.28.1.2 The Implicit Page Trap
...............................

If, after starting GNU 'troff' without loading a macro package, you use
the 'ptr' request to dump a list of the active traps to the standard
error stream,(1) (*note The Implicit Page Trap-Footnote-1::) nothing is
reported.  Yet the '.t' register will report a steadily decreasing value
with every output line your document produces, and once the value of
'.t' gets to within '.V' of zero, you will notice that something
trap-like happens--the page is ejected, a new one begins, and the value
of '.t' becomes large once more.

   This "implicit page trap" always exists in the top-level
diversion;(2) (*note The Implicit Page Trap-Footnote-2::) it works like
a trap in some ways but not others.  Its purpose is to eject the current
page and start the next one.  It has no name, so it cannot be moved or
deleted with 'wh' or 'ch' requests.  You cannot hide it by placing
another trap at its location, and can move it only by redefining the
page length with 'pl'.  Its operation is suppressed when vertical page
traps are disabled with GNU 'troff''s 'vpt' request.

   (1) *Note Debugging::.

   (2) *Note Diversions::.

5.28.1.3 Diversion Traps
........................

A diversion is not formatted in the context of a page, so it lacks page
location traps; instead it can have a "diversion trap".  There can exist
at most one such vertical position trap per diversion.

 -- Request: .dt [dist name]
     Set a trap _within_ a diversion at location DIST, which is
     interpreted relative to diversion rather than page boundaries.  If
     invoked with fewer than two arguments, any diversion trap in the
     current diversion is removed.  The register '.t' works within
     diversions.  It is an error to invoke 'dt' in the top-level
     diversion.  *Note Diversions::.

5.28.2 Input Line Traps
-----------------------

 -- Request: .it [n name]
 -- Request: .itc [n name]
     Set an input line trap, calling macro NAME after processing the
     next N productive input lines (recall *note Manipulating Filling
     and Adjustment::).  Any existing input line trap in the environment
     is replaced.  Without arguments, 'it' and 'itc' clear any input
     line trap that has not yet sprung.

     Consider a macro '.ST S N' which sets the next N input lines in the
     font style S.

          .de ST \" Use style $1 for next $2 text lines.
          .  it \\$2 ES
          .  ft \\$1
          ..
          .de ES \" end ST
          .  ft R
          ..
          .ST I 1
          oblique
          face
          .ST I 1
          oblique\c
          face
              => oblique face obliqueface  (second "face" upright)

     Unlike the 'ce' and 'rj' requests, 'it' counts lines interrupted
     with the '\c' escape sequence separately (*note Line
     Continuation::); 'itc' does not.  To see the difference, let's
     change the previous example to use 'itc' instead.

          ...
          .  itc \\$2 ES
          ...
              => oblique face obliqueface  (second "face" oblique)

     You can think of the 'ce' and 'rj' requests as implicitly creating
     an input line trap with 'itc' that schedules a break when the trap
     is sprung.

          .de BR
          .  br
          .  internal: disable centering-without-filling
          ..
          .
          .de ce
          .  if \\n[.br] .br
          .  itc \\$1 BR
          .  internal: enable centering-without-filling
          ..

     Let us consider in more detail the sorts of input lines that are or
     are not "productive".

          .de Trap
          TRAP SPRUNG
          ..
          .de Mac
          .if r a \l'5n'
          ..
          .it 2 Trap
          .
          foo
          .Mac
          bar
          baz
          .it 1 Trap
          .sp \" moves, but does not write or draw
          qux
          .itc 1 Trap
          \h'5n'\c \" moves, but does not write or draw
          jat

     When 'Trap' gets called depends on whether the 'a' register is
     defined; the control line with the 'if' request may or may not
     produce written output.  We also see that the spacing request 'sp',
     while certainly affecting the output, does not spring the input
     line trap.  Similarly, the horizontal motion escape sequence '\h'
     also affected the output, but was not "written".  Observe that we
     had to follow it with '\c' and use 'itc' to prevent the newline at
     the end of the text line from causing a word break, which, like an
     ordinary space character, counts as written output.

          $ groff -Tascii input-trap-example.groff
              => foo bar TRAP SPRUNG baz
              =>
              => qux TRAP SPRUNG      jat TRAP SPRUNG
          $ groff -Tascii -ra1 input-trap-example.groff
              => foo _____ TRAP SPRUNG bar baz
              =>
              => qux TRAP SPRUNG      jat TRAP SPRUNG

   Input line traps are associated with the environment (*note
Environments::); switching to another environment suspends the current
input line trap, and going back resumes it, restoring the count of
qualifying lines enumerated in that environment.

5.28.3 Blank Line Traps
-----------------------

 -- Request: .blm [name]
     Set a blank line trap, calling the macro NAME when GNU 'troff'
     encounters a blank line in an input file, instead of the usual
     behavior (*note Breaking::).  A line consisting only of spaces is
     also treated as blank and subject to this trap.  If no argument is
     supplied, the default blank line behavior is (re-)established.

5.28.4 Leading Space Traps
--------------------------

 -- Request: .lsm [name]
 -- Register: \n[lsn]
 -- Register: \n[lss]
     Set a leading space trap, calling the macro NAME when GNU 'troff'
     encounters leading spaces in an input line; the implicit line break
     that normally happens in this case is suppressed.  If no argument
     is supplied, the default leading space behavior is (re-)established
     (*note Breaking::).

     The count of leading spaces on an input line is stored in register
     'lsn', and the amount of corresponding horizontal motion in
     register 'lss', irrespective of whether a leading space trap is
     set.  When it is, the leading spaces are removed from the input
     line, and no motion is produced before calling NAME.

5.28.5 End-of-input Traps
-------------------------

 -- Request: .em [name]
     Set a trap at the end of input, calling macro NAME after the last
     line of the last input file has been processed.  If no argument is
     given, any existing end-of-input trap is removed.

     For example, if the document had to have a section at the bottom of
     the last page for someone to approve it, the 'em' request could be
     used.

          .de approval
          \c
          .  ne 3v
          .  sp (\\n[.t]u - 3v)
          .  in +4i
          .  lc _
          .  br
          Approved:\t\a
          .  sp
          Date:\t\t\a
          ..
          .
          .em approval

     The '\c' in the above example needs explanation.  For historical
     reasons (compatibility with AT&T 'troff'), the end-of-input macro
     exits as soon as it causes a page break if no partially collected
     line remains.(1)  (*note End-of-input Traps-Footnote-1::)

     Let us assume that there is no '\c' in the above 'approval' macro,
     that the page is full, and last output line has been broken with,
     say, a 'br' request.  Because there is no more room, a 'ne' request
     at this point causes a page ejection, which in turn makes 'troff'
     exit immediately as just described.  In most situations, this is
     not desired; people generally want to format the input after 'ne'.

     To force processing of the whole end-of-input macro independently
     of this behavior, it is thus advisable to (invisibly) ensure the
     existence of a partially collected line ('\c') whenever there is a
     chance that a page break can happen.  In the above example,
     invoking the 'ne' request ensures that there is room for the
     subsequent formatted output on the same page, so we need insert
     '\c' only once.

     The next example shows how to append three lines, then start a new
     page unconditionally.  Since '.ne 1' doesn't give the desired
     effect--there is always one line available or we are already at the
     beginning of the next page--we temporarily increase the page length
     by one line so that we can use '.ne 2'.

          .de EM
          .pl +1v
          \c
          .ne 2
          line one
          .br
          \c
          .ne 2
          line two
          .br
          \c
          .ne 2
          line three
          .br
          .pl -1v
          \c
          'bp
          ..
          .em EM

     This specific feature affects only the first potential page break
     caused by the end-of-input macro; further page breaks emitted by
     the macro are handled normally.

     Another possible use of the 'em' request is to make GNU 'troff'
     emit a single large page instead of multiple pages.  For example,
     one may want to produce a long plain text file for reading in a
     terminal or emulator without page footers and headers interrupting
     the body of the document.  One approach is to set the page length
     at the beginning of the document to a very large value to hold all
     the text,(2) (*note End-of-input Traps-Footnote-2::) and
     automatically adjust it to the exact height of the document after
     the text has been output.

          .de adjust-page-length
          .  br
          .  pl \\n[nl]u \" \n[nl]: current vertical position
          ..
          .
          .de single-page-mode
          .  pl 99999
          .  em adjust-page-length
          ..
          .
          .\" Activate the above code if configured.
          .if \n[do-continuous-rendering] \
          .  single-page-mode

     Since only one end-of-input trap exists and another macro package
     may already use it, care must be taken not to break the mechanism.
     A simple solution would be to append the above macro to the macro
     package's end-of-input macro using the 'am' request.

   (1) While processing an end-of-input macro, the formatter assumes
that the next page break must be the last; it goes into "sudden death
overtime".

   (2) Another, taken by the 'groff' 'man' macros, is to intercept 'ne'
requests and wrap 'bp' ones.

5.29 Diversions
===============

In 'roff' systems it is possible to format text as if for output, but
instead of writing it immediately, one can "divert" the formatted text
into a named storage area.  It is retrieved later by specifying its name
after a control character.  The same name space is used for such
diversions as for strings and macros; see *note Identifiers::.  Such
text is sometimes said to be "stored in a macro", but this coinage
obscures the important distinction between macros and strings on one
hand and diversions on the other; the former store _unformatted_ input
text, and the latter capture _formatted_ output.  Diversions also do not
interpret arguments.  Applications of diversions include "keeps"
(preventing a page break from occurring at an inconvenient place by
forcing a set of output lines to be set as a group), footnotes, tables
of contents, and indices.  For orthogonality it is said that GNU 'troff'
is in the "top-level diversion" if no diversion is active (that is,
formatted output is being "diverted" immediately to the output device).

   Dereferencing an undefined diversion will create an empty one of that
name and cause a warning in category 'mac' to be emitted.  *Note
Warnings::, for information about the enablement and suppression of
warnings.  A diversion does not exist for the purpose of testing with
the 'd' conditional operator until its initial definition ends (*note
Operators in Conditionals::).  The following requests are used to create
and alter diversions.

 -- Request: .di [name]
 -- Request: .da [name]
     Start collecting formatted output in a diversion called NAME.  The
     'da' request appends to a diversion called NAME, creating it if
     necessary.  If NAME already exists as an alias, the target of the
     alias is replaced or appended to; recall *note Strings::.  The
     pending output line is diverted as well.  Switching to another
     environment (with the 'ev' request) before invoking 'di' or 'da'
     avoids including any pending output line in the diversion; see
     *note Environments::.

     Invoking 'di' or 'da' without an argument stops diverting output to
     the diversion named by the most recent corresponding request.  If
     'di' or 'da' is called without an argument when there is no current
     diversion, a warning in category 'di' is produced.  *Note
     Warnings::, for information about the enablement and suppression of
     warnings.

          Before the diversion.
          .di yyy
          In the diversion.
          .br
          .di
          After the diversion.
          .br
              => After the diversion.
          .yyy
              => Before the diversion.  In the diversion.

   GNU 'troff' supports "box" requests to exclude a partially collected
line from a diversion, as this is often desirable.

 -- Request: .box [name]
 -- Request: .boxa [name]
     Divert (or append) output to NAME, similarly to the 'di' and 'da'
     requests, respectively.  Any pending output line is _not_ included
     in the diversion.  Without an argument, stop diverting output; any
     pending output line inside the diversion is discarded.

          Before the box.
          .box xxx
          In the box.
          .br
          Hidden treasure.
          .box
          After the box.
          .br
              => Before the box.  After the box.
          .xxx
              => In the box.

   Apart from pending output line inclusion and the request names that
populate them, boxes are handled exactly as diversions are.  All of the
following 'groff' language elements can be used with them
interchangeably.

 -- Register: \n[.z]
 -- Register: \n[.d]
     Diversions may be nested.  The read-only string-valued register
     '.z' contains the name of the current diversion.  The read-only
     register '.d' contains the current vertical place in the diversion.
     If the input text is not being diverted, '.d' reports the same
     location as the register 'nl'.

 -- Register: \n[.h]
     The read-only register '.h' stores the "high-water mark" on the
     current page or in the current diversion.  It corresponds to the
     text baseline of the lowest line on the page.(1)  (*note
     Diversions-Footnote-1::)

          .tm .h==\n[.h], nl==\n[nl]
              => .h==0, nl==-1
          This is a test.
          .br
          .sp 2
          .tm .h==\n[.h], nl==\n[nl]
              => .h==40, nl==120

     As implied by the example, vertical motion does not produce text
     baselines and thus does not increase the value interpolated by
     '\n[.h]'.

 -- Register: \n[dn]
 -- Register: \n[dl]
     After completing a diversion, the writable registers 'dn' and 'dl'
     contain its vertical and horizontal sizes.  Only the lines just
     processed are counted: for the computation of 'dn' and 'dl', the
     requests 'da' and 'boxa' are handled as if 'di' and 'box' had been
     used, respectively--lines that have been already stored in the
     diversion (box) are not taken into account.

          .\" Center text both horizontally and vertically.
          .\" Macro .(c starts centering mode; .)c terminates it.
          .
          .\" Disable the escape character with .eo so that we
          .\" don't have to double backslashes on the "\n"s.
          .eo
          .de (c
          .  br
          .  ev (c
          .  evc 0
          .  in 0
          .  nf
          .  di @c
          ..
          .de )c
          .  br
          .  ev
          .  di
          .  nr @s (((\n[.t]u - \n[dn]u) / 2u) - 1v)
          .  sp \n[@s]u
          .  ce 1000
          .  @c
          .  ce 0
          .  sp \n[@s]u
          .  br
          .  fi
          .  rr @s
          .  rm @c
          ..
          .ec

 -- Escape sequence: \!anything
 -- Escape sequence: \?anything\?
     "Transparently" embed ANYTHING into the current diversion,
     preventing requests, macro calls, and escape sequences from being
     interpreted when read into a diversion.  This is useful for
     preventing them from taking effect until the diverted text is
     actually output.  The '\!' escape sequence transparently embeds
     input up to and including the end of the line.  The '\?' escape
     sequence transparently embeds input until its own next occurrence.

     ANYTHING may not contain newlines; use '\!' by itself to embed
     newlines in a diversion.  The escape sequence '\?' is also
     recognized in copy mode and turned into a single internal code; it
     is this code that terminates ANYTHING.  Thus the following example
     prints 4.

          .nr x 1
          .nf
          .di d
          \?\\?\\\\?\\\\\\\\nx\\\\?\\?\?
          .di
          .nr x 2
          .di e
          .d
          .di
          .nr x 3
          .di f
          .e
          .di
          .nr x 4
          .f

     Both escape sequences read the data in copy mode.

     If '\!' is used in the top-level diversion, its argument is
     directly embedded into GNU 'troff''s intermediate output.  This can
     be used, for example, to control a postprocessor that processes the
     data before it is sent to an output driver.

     The '\?' escape used in the top-level diversion produces no output
     at all; its argument is simply ignored.

 -- Request: .output contents
     Emit CONTENTS directly to GNU 'troff''s intermediate output
     (subject to copy mode interpretation); this is similar to '\!' used
     at the top level.  An initial neutral double quote in CONTENTS is
     stripped to allow embedding of leading spaces.

     This request can't be used before the first page has started--if
     you get an error, simply insert '.br' before the 'output' request.

     Use with caution!  It is normally only needed for mark-up used by a
     postprocessor that does something with the output before sending it
     to the output device, filtering out CONTENTS again.

 -- Request: .asciify div
     "Unformat" the diversion DIV in a way such that Unicode basic Latin
     (ASCII) characters, characters translated with the 'trin' request,
     space characters, and some escape sequences, that were formatted
     and diverted into DIV are treated like ordinary input characters
     when DIV is reread.  Doing so can be useful in conjunction with the
     'writem' request.  'asciify' can be also used for gross hacks; for
     example, the following sets register 'n' to 1.

          .tr @.
          .di x
          @nr n 1
          .br
          .di
          .tr @@
          .asciify x
          .x

     'asciify' cannot return all items in a diversion to their source
     equivalent: nodes such as those produced by the '\N' escape
     sequence will remain nodes, so the result cannot be guaranteed to
     be a pure string.  *Note Copy Mode::.  Glyph parameters such as the
     type face and size are not preserved; use 'unformat' to achieve
     that.

 -- Request: .unformat div
     Like 'asciify', unformat the diversion DIV.  However, 'unformat'
     handles only tabs and spaces between words, the latter usually
     arising from spaces or newlines in the input.  Tabs are treated as
     input tokens, and spaces become adjustable again.  The vertical
     sizes of lines are not preserved, but glyph information (font, type
     size, space width, and so on) is retained.

   (1) Thus, the "water" gets "higher" proceeding _down_ the page.

5.30 Punning Names
==================

Macros, strings, and diversions share a name space; recall *note
Identifiers::.  Internally, the same mechanism is used to store them.
You can thus call a macro with string interpolation syntax and vice
versa.

     .de subject
     Typesetting
     ..
     .de predicate
     rewards attention to detail
     ..
     \*[subject] \*[predicate].
     Truly.
         => Typesetting
         =>  rewards attention to detail Truly.

What went wrong?  Strings don't contain newlines, but macros do.  String
interpolation placed a newline at the end of '\*[subject]', and the next
thing on the input was a space.  Then when '\*[predicate]' was
interpolated, it was followed by the empty request '.' on a line by
itself.  If we want to use macros as strings, we must take interpolation
behavior into account.

     .de subject
     Typesetting\\
     ..
     .de predicate
     rewards attention to detail\\
     ..
     \*[subject] \*[predicate].
     Truly.
         => Typesetting rewards attention to detail.  Truly.

By ending each text line of the macros with an escaped '\<RET>', we get
the desired effect (*note Line Continuation::).(1)  (*note Punning
Names-Footnote-1::) What would have happened if we had used only one
backslash at a time instead?

   Interpolating a string does not hide existing macro arguments.  We
can also place the escaped newline outside the string interpolation
instead of within the string definition.  Thus, in a macro, a more
efficient way of doing

     .xx \\$@

is

     \\*[xx]\\

The latter calling syntax doesn't change the value of '\$0', which is
then inherited from the calling macro (*note Parameters::).

   Diversions can be also called with string syntax.  It is sometimes
convenient to copy one-line diversions to a string.

     .di xx
     the
     .ft I
     interpolation system
     .ft
     .br
     .di
     .ds yy This is a test of \*(xx\c
     \*(yy.
         => This is a test of the interpolation system.

As the previous example shows, it is possible to store formatted output
in strings.  The '\c' escape sequence prevents the subsequent newline
from being interpreted as a break (again, *note Line Continuation::).

   Copying multi-output line diversions produces unexpected results.

     .di xxx
     a funny
     .br
     test
     .br
     .di
     .ds yyy This is \*[xxx]\c
     \*[yyy].
         => test This is a funny.

   Usually, it is not predictable whether a diversion contains one or
more output lines, so this mechanism should be avoided.  With AT&T
'troff', this was the only solution to strip off a final newline from a
diversion.  Another disadvantage is that the spaces in the copied string
are already formatted, preventing their adjustment.  This can cause ugly
results.

   A clean solution to this problem is available in GNU 'troff', using
the requests 'chop' to remove the final newline of a diversion, and
'unformat' to make the horizontal spaces adjustable again.

     .box xxx
     a funny
     .br
     test
     .br
     .box
     .chop xxx
     .unformat xxx
     This is \*[xxx].
         => This is a funny test.

   *Note Gtroff Internals::.

   (1) The backslash is doubled.  *Note Copy Mode::.

5.31 Environments
=================

As discussed in *note Deferring Output::, environments store most of the
parameters that determine the appearance of text.  A default environment
named '0' exists when GNU 'troff' starts up; it is modified by
formatting-related requests and escape sequences.

   You can create new environments and switch among them.  Only one is
current at any given time.  Active environments are managed using a
"stack", a data structure supporting "push" and "pop" operations.  The
current environment is at the top of the stack.  The same environment
name can be pushed onto the stack multiple times, possibly interleaved
with others.  Popping the environment stack does not destroy the current
environment; it remains accessible by name and can be made current again
by pushing it at any time.  Environments cannot be renamed or deleted,
and can only be modified when current.  To inspect the environment
stack, use the 'pev' request; see *note Debugging::.

   Environments store the following information.

   * a partially collected line, if any

   * data about the most recently output glyph and line (registers
     '.cdp', '.cht', '.csk', '.n', '.w')

   * typeface parameters (size, family, style, height and slant,
     inter-word and inter-sentence space sizes)

   * page parameters (line length, title length, vertical spacing, line
     spacing, indentation, line numbering, centering, right-alignment,
     underlining, hyphenation parameters)

   * filling enablement; adjustment enablement and mode

   * tab stops; tab, leader, escape, control, no-break control,
     hyphenation, and margin characters

   * input line traps

   * stroke and fill colors

 -- Request: .ev [ident]
 -- Register: \n[.ev]
     Enter the environment IDENT, which is created if it does not
     already exist, using the same parameters as for the default
     environment used at startup.  With no argument, GNU 'troff'
     switches to the previous environment.

     Invoking 'ev' with an argument puts environment IDENT onto the top
     of the environment stack.  (If it isn't already present in the
     stack, this is a proper push.)  Without an argument, 'ev' pops the
     environment stack, making the previous environment current.  It is
     an error to pop the environment stack with no previous environment
     available.  The read-only string-valued register '.ev' contains the
     name of the current environment--the one at the top of the stack.

          .ev footnote-env
          .fam N
          .ps 6
          .vs 8
          .ll -.5i
          .ev

          ...

          .ev footnote-env
          \[dg] Observe the smaller text and vertical spacing.
          .ev

     We can familiarize ourselves with stack behavior by wrapping the
     'ev' request with a macro that reports the contents of the '.ev'
     register to the standard error stream.

          .de EV
          .  ev \\$1
          .  tm environment is now \\n[.ev]
          ..
          .
          .EV foo
          .EV bar
          .EV
          .EV baz
          .EV
          .EV
          .EV

              error-> environment is now foo
              error-> environment is now bar
              error-> environment is now foo
              error-> environment is now baz
              error-> environment is now foo
              error-> environment is now 0
              error-> error: environment stack underflow
              error-> environment is now 0

 -- Request: .evc environment
     Copy the contents of ENVIRONMENT to the current environment.

     The following environment data are not copied.

        * a partially collected line, if present;

        * the interruption status of the previous input line (due to use
          of the '\c' escape sequence);

        * the count of remaining lines to center, to right-justify, or
          to underline (with or without underlined spaces)--these are
          set to zero;

        * the activation status of temporary indentation;

        * input line traps and their associated data;

        * the activation status of line numbering (which can be
          reactivated with '.nm +0'); and

        * the count of consecutive hyphenated lines (set to zero).

 -- Register: \n[.w]
 -- Register: \n[.cht]
 -- Register: \n[.cdp]
 -- Register: \n[.csk]
     The '\n[.w]' register contains the width of the last glyph
     formatted in the environment.

     The '\n[.cht]' register contains the height of the last glyph
     formatted in the environment.

     The '\n[.cdp]' register contains the depth of the last glyph
     formatted in the environment.  It is positive for glyphs extending
     below the baseline.

     The '\n[.csk]' register contains the "skew" (how far to the right
     of the glyph's center that GNU 'troff' should place an accent) of
     the last glyph formatted in the environment.

 -- Register: \n[.n]
     The '\n[.n]' register contains the length of the previous output
     line emitted in the environment.

5.32 Suppressing Output
=======================

 -- Escape sequence: \O[num]
     Suppress GNU 'troff' output of glyphs and geometric objects.  The
     sequences '\O2', '\O3', '\O4', and '\O5' are intended for internal
     use by 'grohtml'.

     '\O0'
          Disable the emission of glyphs and geometric objects to the
          output driver, provided that this sequence occurs at the
          outermost suppression level (see '\O3' and '\04' below).
          Horizontal motions corresponding to non-overstruck glyph
          widths still occur.

     '\O1'
          Enable the emission of glyphs and geometric objects to the
          output driver, provided that this sequence occurs at the
          outermost suppression level.

     '\O0' and '\O1' also reset the four registers 'opminx', 'opminy',
     'opmaxx', and 'opmaxy' to -1.  These four registers mark the top
     left and bottom right hand corners of a box encompassing all
     written or drawn output.

     '\O2'
          At the outermost suppression level, enable emission of glyphs
          and geometric objects, and write to the standard error stream
          the page number and values of the four aforementioned
          registers encompassing glyphs written since the last
          interpolation of a '\O' sequence, as well as the page offset,
          line length, image file name (if any), horizontal and vertical
          device motion quanta, and input file name.  Numeric values are
          in basic units.

     '\O3'
          Begin a nested suppression level.  'grohtml' uses this
          mechanism to create images of output preprocessed with 'gpic',
          'geqn', and 'gtbl'.  At startup, GNU 'troff' is at the
          outermost suppression level.  'pre-grohtml' generates these
          sequences when processing the document, using GNU 'troff' with
          the 'ps' output device, Ghostscript, and the PNM tools to
          produce images in PNG format.  They start a new page if the
          device is not 'html' or 'xhtml', to reduce the number of
          images crossing a page boundary.

     '\O4'
          End a nested suppression level.

     '\O[5PFILE]'
          At the outermost suppression level, write the name 'file' to
          the standard error stream at position P, which must be one of
          'l', 'r', 'c', or 'i', corresponding to left, right, centered,
          and inline alignments within the document, respectively.  FILE
          is a name associated with the production of the next image.

 -- Register: \n[.O]
     Output suppression nesting level applied by '\O3' and '\O4' escape
     sequences.

5.33 I/O
========

'gtroff' has several requests for including files:

 -- Request: .so file
 -- Request: .soquiet file
     Replace the 'so' request's control line with the contents of the
     file named by the argument, "sourcing" it.  FILE is sought in the
     directories specified by '-I' command-line option.  If FILE does
     not exist, a warning in category 'file' is produced and the request
     has no further effect.  *Note Warnings::, for information about the
     enablement and suppression of warnings.

     'so' can be useful for large documents; e.g., allowing each chapter
     of a book to be kept in a separate file.  However, files
     interpolated with 'so' are not preprocessed; to overcome this
     limitation, see the 'gsoelim(1)' man page.

     Since GNU 'troff' replaces the entire control line with the
     contents of a file, it matters whether 'file' is terminated with a
     newline or not.  Assume that file 'xxx' contains only the word
     'foo' without a trailing newline.

          $ printf 'foo' > xxx

          The situation is
          .so xxx
          bar.
              => The situation is foobar.

     'soquiet' works the same way, except that no warning diagnostic is
     issued if FILE does not exist.

 -- Request: .pso command
     Read the standard output from the specified COMMAND and include it
     in place of the 'pso' request.

     It is an error to use this request in safer mode, which is the
     default.  Invoke GNU 'troff' or a front end with the '-U' option to
     enable unsafe mode.

     The comment regarding a final newline for the 'so' request is valid
     for 'pso' also.

 -- Request: .mso file
 -- Request: .msoquiet file
     Identical to the 'so' and 'soquiet' requests, respectively, except
     that 'gtroff' searches for the specified FILE in the same
     directories as macro files for the '-m' command-line option.  If
     the file name to be included has the form 'NAME.tmac' and it isn't
     found, these requests try to include 'tmac.NAME' and vice versa.

 -- Request: .trf file
 -- Request: .cf file
     Transparently output the contents of FILE.  Each line is output as
     if it were preceded by '\!'; however, the lines are _not_ subject
     to copy mode interpretation.  If the file does not end with a
     newline, 'trf' adds one.  Both requests cause a break.

     When used in a diversion, these requests embed a node (*note Gtroff
     Internals::) in it that, when reread, causes the contents of FILE
     to be transparently copied to the output.  In AT&T 'troff', the
     contents of FILE are immediately copied to the output regardless of
     whether there is a current diversion; this behaviour is so
     anomalous that it must be considered a bug.

     While 'cf' copies the contents of FILE completely unprocessed,
     'trf' disallows characters such as NUL that are not valid 'gtroff'
     input characters (*note Identifiers::).

     For 'cf', within a diversion, "completely unprocessed" means that
     each line of a file to be inserted is handled as if it were
     preceded by '\!\\!'.

     To define a macro 'x' containing the contents of file 'f', use

          .ev 1
          .di x
          .trf f
          .di
          .ev

     The calls to 'ev' prevent the partially collected output line from
     becoming part of the diversion (*note Diversions::).

 -- Request: .nx [file]
     Force 'gtroff' to continue processing of the file specified as an
     argument.  If no argument is given, immediately jump to the end of
     file.

 -- Request: .rd [prompt [arg1 arg2 ...]]
     Read from standard input, and include what is read as though it
     were part of the input file.  Text is read until a blank line is
     encountered.

     If standard input is a TTY input device (keyboard), write PROMPT to
     standard error, followed by a colon (or send BEL for a beep if no
     argument is given).

     Arguments after PROMPT are available for the input.  For example,
     the line

          .rd data foo bar

     with the input 'This is \$2.' prints

          This is bar.

   Using the 'nx' and 'rd' requests, it is easy to set up form letters.
The form letter template is constructed like this, putting the following
lines into a file called 'repeat.let':

     .ce
     \*(td
     .sp 2
     .nf
     .rd
     .sp
     .rd
     .fi
     Body of letter.
     .bp
     .nx repeat.let

When this is run, a file containing the following lines should be
redirected in.  Requests included in this file are executed as though
they were part of the form letter.  The last block of input is the 'ex'
request, which tells GNU 'troff' to stop processing.  If this were not
there, 'troff' would not know when to stop.

     Trent A. Fisher
     708 NW 19th Av., #202
     Portland, OR  97209

     Dear Trent,

     Len Adollar
     4315 Sierra Vista
     San Diego, CA  92103

     Dear Mr. Adollar,

     .ex

 -- Request: .pi pipe
     Pipe the output of 'gtroff' to the shell command(s) specified by
     PIPE.  This request must occur before 'gtroff' has a chance to
     print anything.

     It is an error to use this request in safer mode, which is the
     default.  Invoke GNU 'troff' or a front end with the '-U' option to
     enable unsafe mode.

     Multiple calls to 'pi' are allowed, acting as a chain.  For
     example,

          .pi foo
          .pi bar
          ...

     is the same as '.pi foo | bar'.

     The intermediate output format of GNU 'troff' is piped to the
     specified commands.  Consequently, calling 'groff' without the '-Z'
     option normally causes a fatal error.

 -- Request: .sy cmds
 -- Register: \n[systat]
     Execute the shell command(s) specified by CMDS.  The output is not
     saved anywhere, so it is up to the user to do so.

     It is an error to use this request in safer mode; this is the
     default.  Give GNU 'troff' or a front end program the '-U' option
     to enable unsafe mode.

     The following code fragment introduces the current time into a
     document.

          .sy perl -e 'printf ".nr H %d\\n.nr M %d\\n.nr S %d\\n",\
                       (localtime(time))[2,1,0]' > /tmp/x\n[$$]
          .so /tmp/x\n[$$]
          .sy rm /tmp/x\n[$$]
          \nH:\nM:\nS

     This works by having the Perl script (run by 'sy') write 'nr'
     requests that set the registers 'H', 'M', and 'S' to a temporary
     file.  The 'roff' document then reads the temporary file using the
     'so' request.

     The registers 'seconds', 'minutes', and 'hours', initialized at
     startup of GNU 'troff', should satisfy most requirements.  Use the
     'af' request to format their values for output.

          .af hours 00
          .af minutes 00
          .af seconds 00
          \n[hours]:\n[minutes]:\n[seconds]
              => 02:17:54

     The writable register 'systat' contains the return value of the
     'system()' function executed by the last 'sy' request.

 -- Request: .open stream file
 -- Request: .opena stream file
     Open the specified FILE for writing and associates the specified
     STREAM with it.

     The 'opena' request is like 'open', but if the file exists, append
     to it instead of truncating it.

     It is an error to use these requests in safer mode; this is the
     default.  Give GNU 'troff' or a front end program the '-U' option
     to enable unsafe mode.

 -- Request: .write stream data
 -- Request: .writec stream data
     Write to the file associated with the specified STREAM.  The stream
     must previously have been the subject of an open request.  The
     remainder of the line is interpreted as the 'ds' request reads its
     second argument: an initial neutral double quote in CONTENTS is
     stripped to allow embedding of leading spaces, and it is read in
     copy mode.

     The 'writec' request is like 'write', but only 'write' appends a
     newline to the data.

 -- Request: .writem stream xx
     Write the contents of the macro or string XX to the file associated
     with the specified STREAM.

     XX is read in copy mode, i.e., already formatted elements are
     ignored.  Consequently, diversions must be unformatted with the
     'asciify' request before calling 'writem'.  Usually, this means a
     loss of information.

 -- Request: .close stream
     Close the specified STREAM; the stream is no longer an acceptable
     argument to the 'write' request.

     Here a simple macro to write an index entry.

          .open idx test.idx
          .
          .de IX
          .  write idx \\n[%] \\$*
          ..
          .
          .IX test entry
          .
          .close idx

 -- Escape sequence: \Ve
 -- Escape sequence: \V(ev
 -- Escape sequence: \V[env]
     Interpolate the contents of the specified environment variable ENV
     (one-character name E, two-character name EV) as returned by the
     function 'getenv(3)'.  '\V' is interpreted even in copy mode (*note
     Copy Mode::).

5.34 Postprocessor Access
=========================

Two escape sequences and two requests enable documents to pass
information directly to a postprocessor.  These are useful for
exercising device-specific capabilities that the 'groff' language does
not abstract or generalize; examples include the embedding of hyperlinks
and image files.  Device-specific functions are documented in each
output driver's man page, such as 'gropdf(1)', 'grops(1)', or
'grotty(1)'.

 -- Request: .device xxx ...
 -- Escape sequence: \X'xxx ...'
     Embed all XXX arguments into GNU 'troff' output as parameters to a
     device control command 'x X'.  The meaning and interpretation of
     such parameters is determined by the output driver or other
     postprocessor.

     The 'device' request processes its arguments in copy mode (*note
     Copy Mode::).  An initial neutral double quote in CONTENTS is
     stripped to allow embedding of leading spaces.  By contrast, within
     '\X' arguments, the escape sequences '\&', '\)', '\%', and '\:' are
     ignored; '\<SP>' and '\~' are converted to single space characters;
     and '\\' has its escape character stripped.  So that the basic
     Latin subset of the Unicode character set(1) (*note Postprocessor
     Access-Footnote-1::) can be reliably encoded in device control
     commands, seven special character escape sequences ('\-', '\[aq]',
     '\[dq]', '\[ga]', '\[ha]', '\[rs]', and '\[ti]',) are mapped to
     basic Latin characters; see the 'groff_char(7)' man page.  For this
     transformation, character translations and special character
     definitions are ignored.(2)  (*note Postprocessor
     Access-Footnote-2::) The use of any other escape sequence in '\X'
     parameters is normally an error.

     If the 'use_charnames_in_special' directive appears in the output
     device's 'DESC' file, the use of special character escape sequences
     is _not_ an error; they are simply output verbatim (with the
     exception of the seven mapped to Unicode basic Latin characters,
     discussed above).  'use_charnames_in_special' is currently employed
     only by 'grohtml'.

 -- Request: .devicem name
 -- Escape sequence: \Yn
 -- Escape sequence: \Y(nm
 -- Escape sequence: \Y[name]
     This is approximately equivalent to '\X'\*[NAME]'' (one-character
     name N, two-character name NM).  However, the contents of the
     string or macro NAME are not interpreted; also it is permitted for
     NAME to have been defined as a macro and thus contain newlines (it
     is not permitted for the argument to '\X' to contain newlines).
     The inclusion of newlines requires an extension to the AT&T 'troff'
     output format, and confuses drivers that do not know about this
     extension (*note Device Control Commands::).

 -- Request: .tag name
 -- Request: .taga name
     Reserved for internal use.

   (1) that is, ISO 646:1991-IRV or, popularly, "US-ASCII"

   (2) They are bypassed because these parameters are not rendered as
glyphs in the output; instead, they remain abstract characters--in a PDF
bookmark or a URL, for example.

5.35 Miscellaneous
==================

We document here GNU 'troff' features that fit poorly elsewhere.

 -- Request: .nm [start [increment [space [indentation]]]]
 -- Register: \n[ln]
 -- Register: \n[.nm]
     Begin (or, with no arguments, cease) numbering output lines.  START
     assigns the number of the _next_ output line.  Only line numbers
     divisible by INCREMENT are marked (default: '1').  SPACE configures
     the horizontal spacing between the number and the text (default:
     '1').  Any given INDENTATION is applied to the numbers (default:
     '0').  The third and fourth arguments are reckoned in numeral
     widths ('\0').  START must be non-negative and INCREMENT positive.

     The formatter aligns the number to the right in a width of three
     numeral spaces plus INDENTATION, then catenates SPACE and the
     output line.  The line length is _not_ reduced.  Depending on the
     value of the page offset,(1) (*note Miscellaneous-Footnote-1::)
     numbers wider than the allocated space protrude into the left
     margin, or shift the output line to the right.

     Line numbering parameters corresponding to missing arguments are
     not altered.  After numbering is disabled, '.nm +0' resumes it
     using the previously active parameters.

     The parameters of 'nm' are associated with the environment (*note
     Environments::).

     While numbering is enabled, the output line number register 'ln' is
     updated as each line is output, even if no line number is formatted
     with it because it is being skipped (it is not a multiple of
     INCREMENT) or because numbering is suppressed (see the 'nn' request
     below).

     The '.nm' register tracks the enablement status of numbering.
     Temporary suspension of numbering with the 'nn' request does _not_
     alter its value.

          .po 5n
          .ll 44n
          Programming,
          when stripped of all its circumstantial irrelevancies,
          .nm 999 1 1 -4
          boils down to no more and no less than
          .nm +0 3
          very effective thinking so as to avoid unmastered
          .nn 2
          complexity,
          to very vigorous separation of your many
          different concerns.
          .br
          \(em Edsger Dijkstra
          .sp
          .nm 1 1 1
          This guy's arrogance takes your breath away.
          .br
          \(em John Backus
              =>      Programming,  when  stripped of all its cir-
              =>  999 cumstantial irrelevancies, boils down to  no
              =>      more  and no less than very effective think-
              =>      ing so as to avoid unmastered complexity, to
              =>      very vigorous separation of your  many  dif-
              =>      ferent concerns.
              => 1002 -- Edsger Dijkstra
              =>
              =>    1 This guy's arrogance takes your breath away.
              =>    2 -- John Backus

 -- Request: .nn [skip]
 -- Register: \n[.nn]
     Suppress numbering of the next SKIP output lines that would
     otherwise be numbered.  The default is 1.  'nn' can be invoked when
     line numbering is not active; suppression of numbering will take
     effect for SKIP lines once 'nm' enables it.

     The '.nn' register stores the count of output lines still to have
     their numbering suppressed.

     This count is associated with the environment (*note
     Environments::).

   To test whether the current output line will be numbered, you must
check both the '.nm' and '.nn' registers.

       .de is-numbered
       .  nop This line
       .  ie (\\n[.nm] & (1-\\n[.nn])) IS
       .  el                           ISN'T
       .  nop numbered.
       .  br
       ..
       Test line numbering.
       .is-numbered
       .nm 1
       .nn 1
       .is-numbered
       .is-numbered
       .nm
       .is-numbered
         => Test line numbering.  This line ISN'T numbered.
         => This line ISN'T numbered.
         =>   1 This line IS numbered.
         => This line ISN'T numbered.

 -- Request: .mc [margin-character [distance]
     Begin (or, with no arguments, cease) writing a "margin-character"
     to the right of each output line.  The DISTANCE argument separates
     MARGIN-CHARACTER from the right margin.  If absent, the most recent
     value is used; the default is 10 points.  If an output line exceeds
     the line length, the margin character is appended to it.  No margin
     character is written on lines produced by the 'tl' request.

     The margin character is a property of the output line; the margin
     character last configured when the line is output controls.  If the
     margin character is disabled before an output line breaks, none is
     output (but see below).

     The margin character is associated with the environment (*note
     Environments::).

          .ll 5i
          .nf
          .mc \[br]
          This paragraph is marked with a margin character.
          .sp
          As seen above, vertical space isn't thus marked.
          \&
          An output line that is present, but empty, is.
              => This paragraph is marked with a margin character.  |
              =>
              => As seen above, vertical space isn't thus marked.   |
              =>                                                    |
              => An output line that is present, but empty, is.     |

   For compatibility with AT&T 'troff', a call to 'mc' to set the margin
character can't be undone immediately; at least one line gets a margin
character.

     .ll 10n
     .nf
     .mc |
     .mc *
     .mc
     foo
     bar
         => foo        *
         => bar

   The margin character mechanism is commonly used to annotate changes
in documents.  The 'groff' distribution ships a program, 'gdiffmk', to
assist with this task.(2)  (*note Miscellaneous-Footnote-2::)

 -- Request: .psbb file
 -- Register: \n[llx]
 -- Register: \n[lly]
 -- Register: \n[urx]
 -- Register: \n[ury]
     Retrieve the bounding box of the PostScript image found in FILE,
     which must conform to Adobe's "Document Structuring Conventions"
     (DSC), locate a '%%BoundingBox' comment, and store the (upper-,
     lower-, -left, -right) values into the registers 'llx', 'lly',
     'urx', and 'ury'.  If an error occurs (for example, if no
     '%%BoundingBox' comment is present), the formatter sets these
     registers to 0.

     The search path for FILE can be controlled with the '-I'
     command-line option.

   (1) Recall *note Line Layout::.

   (2) Historically, tools named 'nrchbar' and 'changebar' were
developed for marking changes with margin characters and could be found
in archives of the 'comp.sources.unix' USENET group.  Some proprietary
Unices also offer(ed) a 'diffmk' program.

5.36 'gtroff' Internals
=======================

'gtroff' processes input in three steps.  One or more input characters
are converted to an "input token".(1)  (*note Gtroff
Internals-Footnote-1::) Then, one or more input tokens are converted to
an "output node".  Finally, output nodes are converted to the
intermediate output language understood by all output devices.

   Actually, before step one happens, 'gtroff' converts certain escape
sequences into reserved input characters (not accessible by the user);
such reserved characters are used for other internal processing also -
this is the very reason why not all characters are valid input.  *Note
Identifiers::, for more on this topic.

   For example, the input string 'fi\[:u]' is converted into a character
token 'f', a character token 'i', and a special token ':u' (representing
u umlaut).  Later on, the character tokens 'f' and 'i' are merged to a
single output node representing the ligature glyph 'fi' (provided the
current font has a glyph for this ligature); the same happens with ':u'.
All output glyph nodes are 'processed', which means that they are
invariably associated with a given font, font size, advance width, etc.
During the formatting process, 'gtroff' itself adds various nodes to
control the data flow.

   Macros, diversions, and strings collect elements in two chained
lists: a list of input tokens that have been passed unprocessed, and a
list of output nodes.  Consider the following diversion.

     .di xxx
     a
     \!b
     c
     .br
     .di

It contains these elements.

node list            token list   element number
                                  
line start node      --           1
glyph node 'a'       --           2
word space node      --           3
--                   'b'          4
--                   '\n'         5
glyph node 'c'       --           6
vertical size node   --           7
vertical size node   --           8
--                   '\n'         9

Elements 1, 7, and 8 are inserted by 'gtroff'; the latter two (which are
always present) specify the vertical extent of the last line, possibly
modified by '\x'.  The 'br' request finishes the pending output line,
inserting a newline input token, which is subsequently converted to a
space when the diversion is reread.  Note that the word space node has a
fixed width that isn't adjustable anymore.  To convert horizontal space
nodes back to input tokens, use the 'unformat' request.

   Macros only contain elements in the token list (and the node list is
empty); diversions and strings can contain elements in both lists.

   The 'chop' request simply reduces the number of elements in a macro,
string, or diversion by one.  Exceptions are "compatibility save" and
"compatibility ignore" input tokens, which are ignored.  The 'substring'
request also ignores those input tokens.

   Some requests like 'tr' or 'cflags' work on glyph identifiers only;
this means that the associated glyph can be changed without destroying
this association.  This can be very helpful for substituting glyphs.  In
the following example, we assume that glyph 'foo' isn't available by
default, so we provide a substitution using the 'fchar' request and map
it to input character 'x'.

     .fchar \[foo] foo
     .tr x \[foo]

Now let us assume that we install an additional special font 'bar' that
has glyph 'foo'.

     .special bar
     .rchar \[foo]

Since glyphs defined with 'fchar' are searched before glyphs in special
fonts, we must call 'rchar' to remove the definition of the fallback
glyph.  Anyway, the translation is still active; 'x' now maps to the
real glyph 'foo'.

   Macro and request arguments preserve compatibility mode enablement.

     .cp 1     \" switch to compatibility mode
     .de xx
     \\$1
     ..
     .cp 0     \" switch compatibility mode off
     .xx caf\['e]
         => caf�

Since compatibility mode is enabled while 'de' is invoked, the macro
'xx' enables compatibility mode when it is called.  Argument '$1' can
still be handled properly because it inherits the compatibility mode
enablement status that was active at the point where 'xx' was called.

   After interpolation of the parameters, the compatibility save and
restore tokens are removed.

   (1) Except the escape sequences '\f', '\F', '\H', '\m', '\M', '\R',
'\s', and '\S', which are processed immediately if not in copy mode.

5.37 Debugging
==============

          Standard troff voodoo, just put a power of two backslashes in
    front of it until it works and if you still have problems add a \c.
                                                         -- Ron Natalie

   GNU 'troff' is not the easiest language to debug, in part thanks to
its design features of recursive interpolation and the use of
multi-stage pipeline processing in the surrounding system.  Nevertheless
there exist several features useful for troubleshooting.

   Preprocessors use the 'lf' request to preserve the identity of the
line numbers and names of input files.  GNU 'troff' emits a variety of
error diagnostics and supports several categories of warning; the output
of these can be selectively suppressed.  A trace of the formatter's
input processing stack can be emitted when errors or warnings occur by
means of GNU 'troff''s '-b' option, or produced on demand with the
'backtrace' request.  The 'tm' and related requests can be used to emit
customized diagnostic messages or for instrumentation while
troubleshooting.  The 'ex' and 'ab' requests cause early termination
with successful and error exit codes respectively, to halt further
processing when continuing would be fruitless.  Examine the state of the
formatter with requests that write lists of defined names (macros,
strings, and diversions), environments, registers, and page location
traps to the standard error stream.

 -- Request: .lf line [file]
     Set the input line number (and, optionally, the file name) GNU
     'troff' shall use for error and warning messages.  LINE is the
     input line number of the _next_ line.  Without an argument, the
     request is ignored.

     'lf''s primary purpose is to aid the debugging of documents that
     undergo preprocessing.  Programs like 'tbl' that transform input in
     their own languages into 'roff' requests use it so that any
     diagnostic messages emitted by 'troff' correspond to the source
     document.

 -- Request: .tm message
 -- Request: .tm1 message
 -- Request: .tmc message
     Send MESSAGE, which consumes the remainder of the input line and
     cannot contain special characters, to the standard error stream,
     followed by a newline.  Leading spaces in MESSAGE are ignored.

     'tm1' is similar, but recognizes and strips a leading neutral
     double quote from MESSAGE to allow the embedding of leading spaces.

     'tmc' works as 'tm1', but does not append a newline.

 -- Request: .ab [message]
     Write any MESSAGE to the standard error stream (like 'tm') and then
     abort GNU 'troff'; that is, stop processing and terminate with a
     failure status.

 -- Request: .ex
     Exit GNU 'troff'; that is, stop processing and terminate with a
     successful status.  To stop processing only the current file, use
     the 'nx' request; see *note I/O::.

   When doing something involved, it is useful to leave the debugging
statements in the code and have them turned on by a command-line flag.

     .if \n[DB] .tm debugging output

To activate such statements, use the '-r' option to set the register.

     groff -rDB=1 file

   If it is known in advance that there are many errors and no useful
output, GNU 'troff' can be forced to suppress formatted output with the
'-z' option.

 -- Request: .pev
     Report the state of the current environment followed by that of all
     other environments to the standard error stream.

 -- Request: .pm
     Report, to the standard error stream, the names of all defined
     macros, strings, and diversions with their sizes in bytes.

 -- Request: .pnr
     Report the names and contents of all currently defined registers to
     the standard error stream.

 -- Request: .ptr
     Report the names and positions of all page location traps to the
     standard error stream.  Empty slots in the list, where a trap has
     been planted but subsequently (re)moved, are printed as well.

 -- Request: .fl
     Instruct 'gtroff' to flush its output immediately.  The intent is
     for interactive use, but this behaviour is currently not
     implemented in 'gtroff'.  Contrary to Unix 'troff', TTY output is
     sent to a device driver also ('grotty'), making it non-trivial to
     communicate interactively.

     This request causes a line break.

 -- Request: .backtrace
     Write the state of the input stack to the standard error stream.

     Consider the following in a file 'test'.

          .de xxx
          .  backtrace
          ..
          .de yyy
          .  xxx
          ..
          .
          .yyy
              error-> troff: backtrace: 'test':2: macro 'xxx'
              error-> troff: backtrace: 'test':5: macro 'yyy'
              error-> troff: backtrace: file 'test':8

     The '-b' option of GNU 'troff' causes a backtrace to be generated
     on each error or warning.  Some warnings have to be enabled; *Note
     Warnings::.

 -- Register: \n[slimit]
     If greater than 0, sets the maximum quantity of objects on GNU
     'troff''s internal input stack.  If less than or equal to 0, there
     is no limit: recursion can continue until program memory is
     exhausted.  The default is 1,000.

 -- Request: .warnscale su
     Set the scaling unit used in certain warnings to SU, which can take
     the values 'u', 'i', 'c', 'p', and 'P'.  The default is 'i'.

 -- Request: .spreadwarn [limit]
     Emit a 'break' warning if the additional space inserted for each
     space between words in an output line adjusted to both margins with
     '.ad b' is larger than or equal to LIMIT.  A negative value is
     treated as zero; an absent argument toggles the warning on and off
     without changing LIMIT.  The default scaling unit is 'm'.  At
     startup, 'spreadwarn' is inactive and LIMIT is 3m.

     For example,

          .spreadwarn 0.2m

     causes a warning if 'break' warnings are not suppressed and
     'gtroff' must add 0.2m or more for each inter-word space in a line.
     *Note Warnings::.

   GNU 'troff' has command-line options for reporting warnings ('-w')
and backtraces ('-b') when a warning or an error occurs.

 -- Request: .warn [n]
 -- Register: \n[.warn]
     Select the categories, or "types", of reported warnings.  N is the
     sum of the numeric codes associated with each warning category that
     is to be enabled; all other categories are disabled.  The
     categories and their associated codes are listed in *note
     Warnings::.  For example, '.warn 0' disables all warnings, and
     '.warn 1' disables all warnings except those about missing glyphs.
     If no argument is given, all warning categories are enabled.

     The read-only register '.warn' contains the sum of the numeric
     codes of enabled warning categories.

5.37.1 Warnings
---------------

Warning diagnostics emitted by GNU 'troff' are divided into named,
numbered categories.  The name associated with each warning category is
used by the '-w' and '-W' options.  Each category is also assigned a
power of two; the sum of enabled category values is used by the 'warn'
request and the '.warn' register.

   Warnings of each category are produced under the following
circumstances.

'char'
'1'
     No mounted font defines a glyph for the requested character.  This
     category is enabled by default.

'number'
'2'
     An invalid numeric expression was encountered.  This category is
     enabled by default.  *Note Numeric Expressions::.

'break'
'4'
     A filled output line could not be broken such that its length was
     less than the output line length '\n[.l]'.  This category is
     enabled by default.

'delim'
'8'
     The closing delimiter in an escape sequence was missing or
     mismatched.

'el'
'16'
     The 'el' request was encountered with no prior corresponding 'ie'
     request.  *Note if-else::.

'scale'
'32'
     A scaling unit inappropriate to its context was used in a numeric
     expression.

'range'
'64'
     A numeric expression was out of range for its context.

'syntax'
'128'
     A self-contradictory hyphenation mode was requested; an empty or
     incomplete numeric expression was encountered; an operand to a
     numeric operator was missing; an attempt was made to define a
     recursive, empty, or nonsensical character class; or a 'groff'
     extension conditional expression operator was used while in
     compatibility mode.

'di'
'256'
     A 'di', 'da', 'box', or 'boxa' request was invoked without an
     argument when there was no current diversion.

'mac'
'512'
     An undefined string, macro, or diversion was used.  When such an
     object is dereferenced, an empty one of that name is automatically
     created.  So, unless it is later deleted, at most one warning is
     given for each.

     This warning is also emitted upon an attempt to move an unplanted
     trap macro (*note Page Location Traps::).  In such cases, the
     unplanted macro is _not_ dereferenced, so it is not created if it
     does not exist.

'reg'
'1024'
     An undefined register was used.  When an undefined register is
     dereferenced, it is automatically defined with a value of 0.  So,
     unless it is later deleted, at most one warning is given for each.

'tab'
'2048'
     A tab character was encountered where a number was expected, or
     appeared in an unquoted macro argument.

'right-brace'
'4096'
     A right brace escape sequence '\}' was encountered where a number
     was expected.

'missing'
'8192'
     A request was invoked with a mandatory argument absent.

'input'
'16384'
     An invalid character occurred on the input stream.

'escape'
'32768'
     An unsupported escape sequence was encountered.

'space'
'65536'
     A space was missing between a request or macro and its argument.
     This warning is produced when an undefined name longer than two
     characters is encountered and the first two characters of the name
     constitute a defined name.  No request is invoked, no macro called,
     and an empty macro is not defined.  This category is enabled by
     default.  It never occurs in compatibility mode.

'font'
'131072'
     A non-existent font was selected, or the selection was ignored
     because a font selection escape sequence was used after the output
     line continuation escape sequence on an input line.  This category
     is enabled by default.

'ig'
'262144'
     An invalid escape sequence occurred in input ignored using the 'ig'
     request.  This warning category diagnoses a condition that is an
     error when it occurs in non-ignored input.

'color'
'524288'
     An undefined color was selected, an attempt was made to define a
     color using an unrecognized color space, an invalid component in a
     color definition was encountered, or an attempt was made to
     redefine a default color.

'file'
'1048576'
     An attempt was made to load a file that does not exist.  This
     category is enabled by default.

   Two warning names group other warning categories for convenience.

'all'
     All warning categories except 'di', 'mac' and 'reg'.  This
     shorthand is intended to produce all warnings that are useful with
     macro packages written for AT&T 'troff' and its descendants, which
     have less fastidious diagnostics than GNU 'troff'.

'w'
     All warning categories.  Authors of documents and macro packages
     targeting 'groff' are encouraged to use this setting.

5.38 Implementation Differences
===============================

GNU 'troff' has a number of features that cause incompatibilities with
documents written for other versions of 'troff'.  Some GNU extensions to
'troff' have become supported by other implementations.

5.38.1 Safer Mode
-----------------

The formatter operates in "safer" mode by default; to mitigate risks
from untrusted input documents, the 'pi' and 'sy' requests are disabled.
GNU 'troff''s '-U' option enables "unsafe mode", restoring their
function and enabling additional 'groff' extension requests, 'open',
'opena', and 'pso'.  *Note I/O::.

5.38.2 Compatibility Mode
-------------------------

Long identifier names may be GNU 'troff''s most obvious innovation.
AT&T 'troff' interprets '.dsabcd' as defining a string 'ab' with
contents 'cd'.  Normally, GNU 'troff' interprets this as a call of a
macro named 'dsabcd'.  AT&T 'troff' also interprets '\*[' and '\n[' as
an interpolation of a string or register, respectively, named '['.  In
GNU 'troff', however, the '[' is normally interpreted as delimiting a
long name.  In compatibility mode, GNU 'troff' interprets names in the
traditional way; they thus can be two characters long at most.

 -- Request: .cp [n]
 -- Register: \n[.C]
     If N is missing or non-zero, turn on compatibility mode; otherwise,
     turn it off.

     The read-only register '.C' is 1 if compatibility mode is on,
     0 otherwise.

     Compatibility mode can be also turned on with the '-C' command-line
     option.

 -- Request: .do name
 -- Register: \n[.cp]
     The 'do' request interprets the string, request, diversion, or
     macro NAME (along with any further arguments) with compatibility
     mode disabled.  Compatibility mode is restored (only if it was
     active) when the _expansion_ of NAME is interpreted; that is, the
     restored compatibility state applies to the contents of the macro,
     string, or diversion NAME as well as data read from files or pipes
     if NAME is any of the 'so', 'soquiet', 'mso', 'msoquiet', or 'pso'
     requests.

     The following example illustrates several aspects of 'do' behavior.

          .de mac1
          FOO
          ..
          .de1 mac2
          groff
          .mac1
          ..
          .de mac3
          compatibility
          .mac1
          ..
          .de ma
          \\$1
          ..
          .cp 1
          .do mac1
          .do mac2 \" mac2, defined with .de1, calls "mac1"
          .do mac3 \" mac3 calls "ma" with argument "c1"
          .do mac3 \[ti] \" groff syntax accepted in .do arguments
              => FOO groff FOO compatibility c1 ~

     The read-only register '.cp', meaningful only when dereferenced
     from a 'do' request, is 1 if compatibility mode was on when the
     'do' request was encountered, and 0 if it was not.  This register
     is specialized and may require a statement of rationale.

     When writing macro packages or documents that use GNU 'troff'
     features and which may be mixed with other packages or documents
     that do not--common scenarios include serial processing of man
     pages or use of the 'so' or 'mso' requests--you may desire correct
     operation regardless of compatibility mode enablement in the
     surrounding context.  It may occur to you to save the existing
     value of '\n(.C' into a register, say, '_C', at the beginning of
     your file, turn compatibility mode off with '.cp 0', then restore
     it from that register at the end with '.cp \n(_C'.  At the same
     time, a modular design of a document or macro package may lead you
     to multiple layers of inclusion.  You cannot use the same register
     name everywhere lest you "clobber" the value from a preceding or
     enclosing context.  The two-character register name space of AT&T
     'troff' is confining and mnemonically challenging; you may wish to
     use the more capacious name space of GNU 'troff'.  However,
     attempting '.nr _my_saved_C \n(.C' will not work in compatibility
     mode; the register name is too long.  "This is exactly what 'do' is
     for," you think, '.do nr _my_saved_C \n(.C'.  The foregoing will
     always save zero to your register, because 'do' turns compatibility
     mode _off_ while it interprets its argument list.

     To robustly save compatibility mode before switching it off, use

          .do nr _my_saved_C \n[.cp]
          .cp 0

     at the beginning of your file, followed by

          .cp \n[_my_saved_C]
          .do rr _my_saved_C

     at the end.  As in the C language, we all have to share one big
     name space, so choose a register name that is unlikely to collide
     with other uses.

   Normally, GNU 'troff' preserves the interpolation depth in delimited
arguments, but not in compatibility mode.

     .ds xx '
     \w'abc\*(xxdef'
         => 168 (normal mode on a terminal device)
         => 72def' (compatibility mode on a terminal device)

   Furthermore, the escape sequences '\f', '\H', '\m', '\M', '\R', '\s',
and '\S' are transparent for the purpose of recognizing a control
character at the beginning of a line only in compatibility mode.  For
example, this code produces bold output in both cases, but the text
differs.

     .de xx
     Hello!
     ..
     \fB.xx\fP
         => .xx (normal mode)
         => Hello! (compatibility mode)

   Normally, the syntax form '\s'N accepts only a single character (a
digit) for N, consistently with other forms that originated in AT&T
'troff', like '\*', '\$', '\f', '\g', '\k', '\n', and '\z'.  In
compatibility mode only, a non-zero N must be in the range 4-39.  Legacy
documents relying upon this quirk of parsing(1) (*note Compatibility
Mode-Footnote-1::) should be migrated to another '\s' form.

   (1) The Graphic Systems C/A/T phototypesetter (the original device
target for AT&T 'troff') supported only a few discrete type sizes in the
range 6-36 points, so Ossanna contrived a special case in the parser to
do what the user must have meant.  Kernighan warned of this in the 1992
revision of CSTR #54 (�2.3), and more recently, McIlroy referred to it
as a "living fossil".

5.38.3 Other Differences
------------------------

'groff' request names unrecognized by other 'troff' implementations will
likely be ignored by them; escape sequences that are 'groff' extensions
are liable to be interpreted as if the escape character were not
present.  For example, the adjustable, non-breaking escape sequence '\~'
is also supported by Heirloom Doctools 'troff' 050915 (September 2005),
'mandoc' 1.9.5 (2009-09-21), 'neatroff' (commit 1c6ab0f6e, 2016-09-13),
and Plan 9 from User Space 'troff' (commit 93f8143600, 2022-08-12), but
not by Solaris or Documenter's Workbench 'troff's.  *Note Manipulating
Filling and Adjustment::.

   GNU 'troff' does not allow the use of the escape sequences '\|',
'\^', '\&', '\{', '\}', '\<SP>', '\'', '\`', '\-', '\_', '\!', '\%', and
'\c' in identifiers; AT&T 'troff' does.  The '\A' escape sequence (*note
Identifiers::) may be helpful in avoiding use of these escape sequences
in names.

   When adjusting to both margins, AT&T 'troff' at first adjusts spaces
starting from the right; GNU 'troff' begins from the left.  Both
implementations adjust spaces from opposite ends on alternating output
lines in this adjustment mode to prevent "rivers" in the text.

   GNU 'troff' does not always hyphenate words as AT&T 'troff' does.
The AT&T implementation uses a set of hard-coded rules specific to
English, while GNU 'troff' uses language-specific hyphenation pattern
files derived from TeX.  Furthermore, in old versions of 'troff' there
was a limited amount of space to store hyphenation exceptions (arguments
to the 'hw' request); GNU 'troff' has no such restriction.

   GNU 'troff' predefines a string '.T' containing the argument given to
the '-T' command-line option, namely the current output device (for
example, 'pdf' or 'utf8').  The existence of this string is a common
feature of post-CSTR #54 'troff's(1) (*note Other
Differences-Footnote-1::) but valid values are specific to each
implementation.

   AT&T 'troff' ignored attempts to remove read-only registers; GNU
'troff' honors such requests.  *Note Built-in Registers::.

   The (read-only) register '.T' interpolates 1 if GNU 'troff' is called
with the '-T' command-line option, and 0 otherwise.  This behavior
differs from AT&T 'troff', which interpolated 1 only if 'nroff' was the
formatter and was called with '-T'.

   AT&T 'troff' and other implementations handle the 'lf' request
differently.  For them, its LINE argument changes the line number of the
_current_ line.

   AT&T 'troff' had only environments named '0', '1', and '2'.  In GNU
'troff', any number of environments may exist, using any valid
identifiers for their names (*note Identifiers::.)

   Fractional type sizes cause one noteworthy incompatibility.  In AT&T
'troff' the 'ps' request ignores scaling units and thus '.ps 10u' sets
the type size to 10 points, whereas in GNU 'troff' it sets the type size
to 10 _scaled_ points.  *Note Using Fractional Type Sizes::.

   The 'ab' request differs from AT&T 'troff': GNU 'troff' writes no
message to the standard error stream if no arguments are given, and it
exits with a failure status instead of a successful one.

   The 'bp' request differs from AT&T 'troff': GNU 'troff' does not
accept a scaling unit on the argument, a page number; the former
(somewhat uselessly) does.

   The 'pm' request differs from AT&T 'troff': GNU 'troff' reports the
sizes of macros, strings, and diversions in bytes and ignores an
argument to report only the sum of the sizes.

   Unlike AT&T 'troff', GNU 'troff' does not ignore the 'ss' request if
the output is a terminal device; instead, the values of minimal
inter-word and additional inter-sentence space are each rounded down to
the nearest multiple of 12.

   In GNU 'troff' there is a fundamental difference between
(unformatted) characters and (formatted) glyphs.  Everything that
affects how a glyph is output is stored with the glyph node; once a
glyph node has been constructed, it is unaffected by any subsequent
requests that are executed, including 'bd', 'cs', 'tkf', 'tr', or 'fp'
requests.  Normally, glyphs are constructed from characters immediately
before the glyph is added to an output line.  Macros, diversions, and
strings are all, in fact, the same type of object; they contain a
sequence of intermixed character and glyph nodes.  Special characters
transform from one to the other: before being added to the output, they
behave as characters; afterward, they are glyphs.  A glyph node does not
behave like a character node when it is processed by a macro: it does
not inherit any of the special properties that the character from which
it was constructed might have had.  For example, the input

     .di x
     \\\\
     .br
     .di
     .x

produces '\\' in GNU 'troff'.  Each pair of backslashes becomes one
backslash _glyph_; the resulting backslashes are thus not interpreted as
escape _characters_ when they are reread as the diversion is output.
AT&T 'troff' _would_ interpret them as escape characters when rereading
them and end up printing one '\'.

   One correct way to obtain a printable backslash in most documents is
to use the '\e' escape sequence; this always prints a single instance of
the current escape character,(2) (*note Other Differences-Footnote-2::)
regardless of whether or not it is used in a diversion; it also works in
both GNU 'troff' and AT&T 'troff'.

   The other correct way, appropriate in contexts independent of the
backslash's common use as a 'troff' escape character--perhaps in
discussion of character sets or other programming languages--is the
character escape '\(rs' or '\[rs]', for "reverse solidus", from its name
in the ECMA-6 (ISO/IEC 646) standard.(3)  (*note Other
Differences-Footnote-3::)

   To store an escape sequence in a diversion that is interpreted when
the diversion is reread, either use the traditional '\!' transparent
output facility, or, if this is unsuitable, the new '\?' escape
sequence.  *Note Diversions:: and *note Gtroff Internals::.

   In the somewhat pathological case where a diversion exists containing
a partially collected line and a partially collected line at the
top-level diversion has never existed, AT&T 'troff' will output the
partially collected line at the end of input; GNU 'troff' will not.

   (1) DWB 3.3, Solaris, Heirloom Doctools, and Plan 9 'troff' all
support it.

   (2) Naturally, if you've changed the escape character, you need to
prefix the 'e' with whatever it is--and you'll likely get something
other than a backslash in the output.

   (3) The 'rs' special character identifier was not defined in AT&T
'troff''s font description files, but is in those of its lineal
descendant, Heirloom Doctools 'troff', as of the latter's 060716 release
(July 2006).

6 File Formats
**************

All files read and written by 'gtroff' are text files.  The following
two sections describe their format.

6.1 'gtroff' Output
===================

This section describes the 'groff' intermediate output format produced
by GNU 'troff'.

   As 'groff' is a wrapper program around GNU 'troff' and automatically
calls an output driver (or "postprocessor"), this output does not show
up normally.  This is why it is called _intermediate_.  'groff' provides
the option '-Z' to inhibit postprocessing such that the produced
intermediate output is sent to standard output just as it is when
calling GNU 'troff' directly.

   Here, the term "troff output" describes what is output by GNU
'troff', while "intermediate output" refers to the language that is
accepted by the parser that prepares this output for the output drivers.
This parser handles whitespace more flexibly than AT&T's implementation
and implements obsolete elements for compatibility; otherwise, both
formats are the same.(1)  (*note gtroff Output-Footnote-1::)

   The main purpose of the intermediate output concept is to facilitate
the development of postprocessors by providing a common programming
interface for all devices.  It has a language of its own that is
completely different from the 'gtroff' language.  While the 'gtroff'
language is a high-level programming language for text processing, the
intermediate output language is a kind of low-level assembler language
by specifying all positions on the page for writing and drawing.

   The intermediate output produced by 'gtroff' is fairly readable,
while output from AT&T 'troff' is rather hard to understand because of
strange habits that are still supported, but not used any longer by
'gtroff'.

   (1) The parser and postprocessor for intermediate output can be found
in the file
'GROFF-SOURCE-DIR/src/libs/libdriver/input.cpp'.

6.1.1 Language Concepts
-----------------------

The fundamental operation of the GNU 'troff' formatter is the
translation of the 'groff' input language into a device-independent form
primarily concerned with what has to be written or drawn at specific
positions on the output device.  This language is simple and imperative.
In the following discussion, the term "command" always refers to this
intermediate output language, and never to the 'groff' language intended
for direct use by document authors.  Intermediate output commands
comprise several categories: glyph output; font, color, and text size
selection; motion of the printing position; page advancement; drawing of
geometric objects; and device control commands, a catch-all for
operations not easily classified as any of the foregoing, such as
directives to start and stop output, identify the intended output
device, or place URL hyperlinks in supported output formats.

6.1.1.1 Separation
..................

AT&T 'troff' output has strange requirements regarding whitespace.  The
'gtroff' output parser, however, is more tolerant, making whitespace
maximally optional.  Such characters, i.e., the tab, space, and newline,
always have a syntactical meaning.  They are never printable because
spacing within the output is always done by positioning commands.

   Any sequence of space or tab characters is treated as a single
"syntactical space".  It separates commands and arguments, but is only
required when there would occur a clashing between the command code and
the arguments without the space.  Most often, this happens when
variable-length command names, arguments, argument lists, or command
clusters meet.  Commands and arguments with a known, fixed length need
not be separated by syntactical space.

   A line break is a syntactical element, too.  Every command argument
can be followed by whitespace, a comment, or a newline character.  Thus
a "syntactical line break" is defined to consist of optional syntactical
space that is optionally followed by a comment, and a newline character.

   The normal commands, those for positioning and text, consist of a
single letter taking a fixed number of arguments.  For historical
reasons, the parser allows stacking of such commands on the same line,
but fortunately, in 'gtroff''s intermediate output, every command with
at least one argument is followed by a line break, thus providing
excellent readability.

   The other commands--those for drawing and device controlling--have a
more complicated structure; some recognize long command names, and some
take a variable number of arguments.  So all 'D' and 'x' commands were
designed to request a syntactical line break after their last argument.
Only one command, 'x X', has an argument that can span several input
lines; all other commands must have all of their arguments on the same
line as the command, i.e., the arguments may not be split by a line
break.

   Empty lines (these are lines containing only space and/or a comment),
can occur everywhere.  They are just ignored.

6.1.1.2 Argument Units
......................

Some commands take integer arguments that are assumed to represent
values in a measurement unit, but the letter for the corresponding
scaling unit is not written with the output command arguments.  Most
commands assume the scaling unit 'u', the basic unit of the device, some
use 'z', the scaled point unit of the device, while others, such as the
color commands, expect plain integers.

   Single characters can have the eighth bit set, as can the names of
fonts and special characters.  The names of characters and fonts can be
of arbitrary length.  A character that is to be printed is always in the
current font.

   A string argument is always terminated by the next whitespace
character (space, tab, or newline); an embedded '#' character is
regarded as part of the argument, not as the beginning of a comment
command.  An integer argument is already terminated by the next
non-digit character, which then is regarded as the first character of
the next argument or command.

6.1.1.3 Document Parts
......................

A correct intermediate output document consists of two parts, the
"prologue" and the "body".

   The task of the prologue is to set the general device parameters
using three exactly specified commands.  'gtroff''s prologue is
guaranteed to consist of the following three lines (in that order):

     x T DEVICE
     x res N H V
     x init

with the arguments set as outlined in *note Device Control Commands::.
The parser for the intermediate output format is able to interpret
additional whitespace and comments as well even in the prologue.

   The body is the main section for processing the document data.
Syntactically, it is a sequence of any commands different from the ones
used in the prologue.  Processing is terminated as soon as the first
'x stop' command is encountered; the last line of any 'gtroff'
intermediate output always contains such a command.

   Semantically, the body is page oriented.  A new page is started by a
'p' command.  Positioning, writing, and drawing commands are always done
within the current page, so they cannot occur before the first 'p'
command.  Absolute positioning (by the 'H' and 'V' commands) is done
relative to the current page; all other positioning is done relative to
the current location within this page.

6.1.2 Command Reference
-----------------------

This section describes all intermediate output commands, both from AT&T
'troff' as well as the 'gtroff' extensions.

6.1.2.1 Comment Command
.......................

'#ANYTHING<end of line>'
     A comment.  Ignore any characters from the '#' character up to the
     next newline character.

     This command is the only possibility for commenting in the
     intermediate output.  Each comment can be preceded by arbitrary
     syntactical space; every command can be terminated by a comment.

6.1.2.2 Simple Commands
.......................

The commands in this subsection have a command code consisting of a
single character, taking a fixed number of arguments.  Most of them are
commands for positioning and text writing.  These commands are tolerant
of whitespace.  Optionally, syntactical space can be inserted before,
after, and between the command letter and its arguments.  All of these
commands are stackable; i.e., they can be preceded by other simple
commands or followed by arbitrary other commands on the same line.  A
separating syntactical space is necessary only when two integer
arguments would clash or if the preceding argument ends with a string
argument.

'C ID<whitespace>'
     Typeset the glyph of the special character ID.  Trailing
     syntactical space is necessary to allow special character names of
     arbitrary length.  The drawing position is not advanced.

'c G'
     Typeset the glyph of the ordinary character C.  The drawing
     position is not advanced.

'f N'
     Select the font mounted at position N.  N cannot be negative.

'H N'
     Horizontally move the drawing position to N basic units from the
     left edge of the page.  N cannot be negative.

'h N'
     Move the drawing position right N basic units.  AT&T 'troff'
     allowed negative N; GNU 'troff' does not produce such values, but
     'groff''s output driver library handles them.

'm COLOR-SCHEME [COMPONENT ...]'
     Select the stroke color using the COMPONENTs in the color space
     SCHEME.  Each COMPONENT is an integer between 0 and 65535.  The
     quantity of components and their meanings vary with each SCHEME.
     This command is a 'groff' extension.

     'mc CYAN MAGENTA YELLOW'
          Use the CMY color scheme with components cyan, magenta, and
          yellow.

     'md'
          Use the default color (no components; black in most cases).

     'mg GRAY'
          Use a grayscale color scheme with a component ranging between
          0 (black) and 65535 (white).

     'mk CYAN MAGENTA YELLOW BLACK'
          Use the CMYK color scheme with components cyan, magenta,
          yellow, and black.

     'mr RED GREEN BLUE'
          Use the RGB color scheme with components red, green, and blue.

'N N'
     Typeset the glyph with index N in the current font.  N is normally
     a non-negative integer.  The drawing position is not advanced.  The
     'html' and 'xhtml' devices use this command with negative N to
     produce unbreakable space; the absolute value of N is taken and
     interpreted in basic units.

'n B A'
     Indicate a break.  No action is performed; the command is present
     to make the output more easily parsed.  The integers B and A
     describe the vertical space amounts before and after the break,
     respectively.  GNU 'troff' issues this command but 'groff''s output
     driver library ignores it.  See 'v' and 'V' below.

'p N'
     Begin a new page, setting its number to N.  Each page is
     independent, even from those using the same number.  The vertical
     drawing position is set to 0.  All positioning, writing, and
     drawing commands are interpreted in the context of a page, so a
     'p' command must precede them.

's N'
     Set type size to N scaled points (unit 'z' in GNU 'troff'.  AT&T
     'troff' used unscaled points 'p' instead; see *note Output Language
     Compatibility::.

't XYZ<whitespace>'
't XYZ DUMMY-ARG<whitespace>'
     Typeset a word XYZ; that is, set a sequence of ordinary glyphs
     named X, Y, Z, ..., terminated by a space character or a line
     break; an optional second integer argument is ignored (this allows
     the formatter to generate an even number of arguments).  Each glyph
     is set at the current drawing position, and the position is then
     advanced horizontally by the glyph's width.  A glyph's width is
     read from its metrics in the font description file, scaled to the
     current type size, and rounded to a multiple of the horizontal
     motion quantum.  Use the 'C' command to emplace glyphs of special
     characters.  The 't' command is a 'groff' extension and is output
     only for devices whose 'DESC' file contains the 'tcommand'
     directive; see *note DESC File Format::.

'u N XYZ<whitespace>'
     Typeset word XYZ with track kerning.  As 't', but after placing
     each glyph, the drawing position is further advanced horizontally
     by N basic units ('u').  The 'u' command is a 'groff' extension and
     is output only for devices whose 'DESC' file contains the
     'tcommand' directive; see *note DESC File Format::.

'V N'
     Vertically move the drawing position to N basic units from the top
     edge of the page.  N cannot be negative.

'v N'
     Move the drawing position down N basic units.  AT&T 'troff' allowed
     negative N; GNU 'troff' does not produce such values, but 'groff''s
     output driver library handles them.

'w'
     Indicate an inter-word space.  No action is performed; the command
     is present to make the output more easily parsed.  Only adjustable,
     breakable inter-word spaces are thus described; those resulting
     from '\~' or horizontal motion escape sequences are not.  GNU
     'troff' issues this command but 'groff''s output driver library
     ignores it.  See 'h' and 'H' above.

6.1.2.3 Graphics Commands
.........................

Each graphics or drawing command in the intermediate output starts with
the letter 'D', followed by one or two characters that specify a
subcommand; this is followed by a fixed or variable number of integer
arguments that are separated by a single space character.  A 'D' command
may not be followed by another command on the same line (apart from a
comment), so each 'D' command is terminated by a syntactical line break.

   'gtroff' output follows the classical spacing rules (no space between
command and subcommand, all arguments are preceded by a single space
character), but the parser allows optional space between the command
letters and makes the space before the first argument optional.  As
usual, each space can be any sequence of tab and space characters.

   Some graphics commands can take a variable number of arguments.  In
this case, they are integers representing a size measured in basic units
'u'.  The arguments called H1, H2, ..., HN stand for horizontal
distances where positive means right, negative left.  The arguments
called V1, V2, ..., VN stand for vertical distances where positive means
down, negative up.  All these distances are offsets relative to the
current location.

   Each graphics command directly corresponds to a similar 'gtroff' '\D'
escape sequence.  *Note Drawing Geometric Objects::.

   Unknown 'D' commands are assumed to be device-specific.  Its
arguments are parsed as strings; the whole information is then sent to
the postprocessor.

   In the following command reference, the syntax element <line break>
means a syntactical line break as defined above.

'D~ H1 V1 H2 V2 ... HN VN<line break>'
     Draw B-spline from current position to offset (H1,V1), then to
     offset (H2,V2), if given, etc., up to (HN,VN).  This command takes
     a variable number of argument pairs; the current position is moved
     to the terminal point of the drawn curve.

'Da H1 V1 H2 V2<line break>'
     Draw arc from current position to (H1,V1)+(H2,V2) with center at
     (H1,V1); then move the current position to the final point of the
     arc.

'DC D<line break>'
'DC D DUMMY-ARG<line break>'
     Draw a solid circle using the current fill color with diameter D
     (integer in basic units 'u') with leftmost point at the current
     position; then move the current position to the rightmost point of
     the circle.  An optional second integer argument is ignored (this
     allows the formatter to generate an even number of arguments).
     This command is a 'gtroff' extension.

'Dc D<line break>'
     Draw circle line with diameter D (integer in basic units 'u') with
     leftmost point at the current position; then move the current
     position to the rightmost point of the circle.

'DE H V<line break>'
     Draw a solid ellipse in the current fill color with a horizontal
     diameter of H and a vertical diameter of V (both integers in basic
     units 'u') with the leftmost point at the current position; then
     move to the rightmost point of the ellipse.  This command is a
     'gtroff' extension.

'De H V<line break>'
     Draw an outlined ellipse with a horizontal diameter of H and a
     vertical diameter of V (both integers in basic units 'u') with the
     leftmost point at current position; then move to the rightmost
     point of the ellipse.

'DF COLOR-SCHEME [COMPONENT ...]<line break>'
     Set fill color for solid drawing objects using different color
     schemes; the analogous command for setting the color of text, line
     graphics, and the outline of graphic objects is 'm'.  The color
     components are specified as integer arguments between 0 and 65535.
     The number of color components and their meaning vary for the
     different color schemes.  These commands are generated by
     'gtroff''s escape sequences '\D'F ...'' and '\M' (with no other
     corresponding graphics commands).  No position changing.  This
     command is a 'gtroff' extension.

     'DFc CYAN MAGENTA YELLOW<line break>'
          Set fill color for solid drawing objects using the CMY color
          scheme, having the 3 color components CYAN, MAGENTA, and
          YELLOW.

     'DFd<line break>'
          Set fill color for solid drawing objects to the default fill
          color value (black in most cases).  No component arguments.

     'DFg GRAY<line break>'
          Set fill color for solid drawing objects to the shade of gray
          given by the argument, an integer between 0 (black) and 65535
          (white).

     'DFk CYAN MAGENTA YELLOW BLACK<line break>'
          Set fill color for solid drawing objects using the CMYK color
          scheme, having the 4 color components CYAN, MAGENTA, YELLOW,
          and BLACK.

     'DFr RED GREEN BLUE<line break>'
          Set fill color for solid drawing objects using the RGB color
          scheme, having the 3 color components RED, GREEN, and BLUE.

'Df N<line break>'
     The argument N must be an integer in the range -32767 to 32767.

     0 <= N <= 1000
          Set the color for filling solid drawing objects to a shade of
          gray, where 0 corresponds to solid white, 1000 (the default)
          to solid black, and values in between to intermediate shades
          of gray; this is obsoleted by command 'DFg'.

     N < 0 or N > 1000
          Set the filling color to the color that is currently being
          used for the text and the outline, see command 'm'.  For
          example, the command sequence

               mg 0 0 65535
               Df -1

          sets all colors to blue.

     No position changing.  This command is a 'gtroff' extension.

'Dl H V<line break>'
     Draw line from current position to offset (H,V) (integers in basic
     units 'u'); then set current position to the end of the drawn line.

'Dp H1 V1 H2 V2 ... HN VN<line break>'
     Draw a polygon line from current position to offset (H1,V1), from
     there to offset (H2,V2), etc., up to offset (HN,VN), and from there
     back to the starting position.  For historical reasons, the
     position is changed by adding the sum of all arguments with odd
     index to the actual horizontal position and the even ones to the
     vertical position.  Although this doesn't make sense it is kept for
     compatibility.  This command is a 'gtroff' extension.

'DP H1 V1 H2 V2 ... HN VN<line break>'
     Draw a solid polygon in the current fill color rather than an
     outlined polygon, using the same arguments and positioning as the
     corresponding 'Dp' command.  This command is a 'gtroff' extension.

'Dt N<line break>'
     Set the current line thickness to N (an integer in basic units 'u')
     if N>0; if N=0 select the smallest available line thickness; if N<0
     set the line thickness proportional to the type size (this is the
     default before the first 'Dt' command was specified).  For
     historical reasons, the horizontal position is changed by adding
     the argument to the actual horizontal position, while the vertical
     position is not changed.  Although this doesn't make sense it is
     kept for compatibility.  This command is a 'gtroff' extension.

6.1.2.4 Device Control Commands
...............................

Each device control command starts with the letter 'x', followed by a
space character (optional or arbitrary space or tab in 'gtroff') and a
subcommand letter or word; each argument (if any) must be preceded by a
syntactical space.  All 'x' commands are terminated by a syntactical
line break; no device control command can be followed by another command
on the same line (except a comment).

   The subcommand is basically a single letter, but to increase
readability, it can be written as a word, i.e., an arbitrary sequence of
characters terminated by the next tab, space, or newline character.  All
characters of the subcommand word but the first are simply ignored.  For
example, 'gtroff' outputs the initialization command 'x i' as 'x init'
and the resolution command 'x r' as 'x res'.

   In the following, the syntax element <line break> means a syntactical
line break (*note Separation::).

'xF NAME<line break>'
     The 'F' stands for FILENAME.

     Use NAME as the intended name for the current file in error
     reports.  This is useful for remembering the original file name
     when 'gtroff' uses an internal piping mechanism.  The input file is
     not changed by this command.  This command is a 'gtroff' extension.

'xf N S<line break>'
     The 'f' stands for FONT.

     Mount font position N (a non-negative integer) with font named S (a
     text word).  *Note Font Positions::.

'xH N<line break>'
     The 'H' stands for HEIGHT.

     Set glyph height to N (a positive integer in scaled points 'z').
     AT&T 'troff' uses the unit points ('p') instead.  *Note Output
     Language Compatibility::.

'xi<line break>'
     The 'i' stands for INIT.

     Initialize device.  This is the third command of the prologue.

'xp<line break>'
     The 'p' stands for PAUSE.

     Parsed but ignored.  The AT&T 'troff' manual documents this command
     as

          pause device, can be restarted

     but GNU 'troff' output drivers do nothing with this command.

'xr N H V<line break>'
     The 'r' stands for RESOLUTION.

     Resolution is N, while H is the minimal horizontal motion, and V
     the minimal vertical motion possible with this device; all
     arguments are positive integers in basic units 'u' per inch.  This
     is the second command of the prologue.

'xS N<line break>'
     The 'S' stands for SLANT.

     Set slant to N (an integer in basic units 'u').

'xs<line break>'
     The 's' stands for STOP.

     Terminates the processing of the current file; issued as the last
     command of any intermediate 'troff' output.

'xt<line break>'
     The 't' stands for TRAILER.

     Generate trailer information, if any.  In GNU 'troff', this is
     ignored.

'xT XXX<line break>'
     The 'T' stands for TYPESETTER.

     Set the name of the output driver to XXX, a sequence of
     non-whitespace characters terminated by whitespace.  The possible
     names correspond to those of 'groff''s '-T' option.  This is the
     first command of the prologue.

'xu N<line break>'
     The 'u' stands for UNDERLINE.

     Configure underlining of spaces.  If N is 1, start underlining of
     spaces; if N is 0, stop underlining of spaces.  This is needed for
     the 'cu' request in 'nroff' mode and is ignored otherwise.  This
     command is a 'gtroff' extension.

'xX ANYTHING<line break>'
     The 'x' stands for X-ESCAPE.

     Send string ANYTHING uninterpreted to the device.  If the line
     following this command starts with a '+' character this line is
     interpreted as a continuation line in the following sense.  The '+'
     is ignored, but a newline character is sent instead to the device,
     the rest of the line is sent uninterpreted.  The same applies to
     all following lines until the first character of a line is not a
     '+' character.  This command is generated by the 'gtroff' escape
     sequence '\X'.  The line-continuing feature is a 'gtroff'
     extension.

6.1.2.5 Obsolete Command
........................

In AT&T 'troff' output, the writing of a single glyph is mostly done by
a very strange command that combines a horizontal move and a single
character giving the glyph name.  It doesn't have a command code, but is
represented by a 3-character argument consisting of exactly 2 digits and
a character.

DDG
     Move right DD (exactly two decimal digits) basic units 'u', then
     print glyph G (represented as a single character).

     In GNU 'troff', arbitrary syntactical space around and within this
     command is allowed.  Only when a preceding command on the same line
     ends with an argument of variable length is a separating space
     obligatory.  In AT&T 'troff', large clusters of these and other
     commands are used, mostly without spaces; this made such output
     almost unreadable.

   For modern high-resolution devices, this command does not make sense
because the width of the glyphs can become much larger than two decimal
digits.  In 'gtroff', this is only used for the devices 'X75', 'X75-12',
'X100', and 'X100-12'.  For other devices, the commands 't' and 'u'
provide a better functionality.

6.1.3 Intermediate Output Examples
----------------------------------

This section presents the intermediate output generated from the same
input for three different devices.  The input is the sentence 'hell
world' fed into 'gtroff' on the command line.

High-resolution device 'ps'

     This is the standard output of 'gtroff' if no '-T' option is given.

          shell> echo "hell world" | groff -Z -T ps

          x T ps
          x res 72000 1 1
          x init
          p1
          x font 5 TR
          f5
          s10000
          V12000
          H72000
          thell
          wh2500
          tw
          H96620
          torld
          n12000 0
          x trailer
          V792000
          x stop

     This output can be fed into 'grops' to get its representation as a
     PostScript file.

Low-resolution device 'latin1'

     This is similar to the high-resolution device except that the
     positioning is done at a minor scale.  Some comments (lines
     starting with '#') were added for clarification; they were not
     generated by the formatter.

          shell> echo "hell world" | groff -Z -T latin1

          # prologue
          x T latin1
          x res 240 24 40
          x init
          # begin a new page
          p1
          # font setup
          x font 1 R
          f1
          s10
          # initial positioning on the page
          V40
          H0
          # write text 'hell'
          thell
          # inform about space, and issue a horizontal jump
          wh24
          # write text 'world'
          tworld
          # announce line break, but do nothing because...
          n40 0
          # ...the end of the document has been reached
          x trailer
          V2640
          x stop

     This output can be fed into 'grotty' to get a formatted text
     document.

AT&T 'troff' output
     Since a computer monitor has a much lower resolution than modern
     printers, the intermediate output for X11 devices can use the
     jump-and-write command with its 2-digit displacements.

          shell> echo "hell world" | groff -Z -T X100

          x T X100
          x res 100 1 1
          x init
          p1
          x font 5 TR
          f5
          s10
          V16
          H100
          # write text with jump-and-write commands
          ch07e07l03lw06w11o07r05l03dh7
          n16 0
          x trailer
          V1100
          x stop

     This output can be fed into 'xditview' or 'gxditview' for
     displaying in X.

     Due to the obsolete jump-and-write command, the text clusters in
     the AT&T 'troff' output are almost unreadable.

6.1.4 Output Language Compatibility
-----------------------------------

The intermediate output language of AT&T 'troff' was first documented in
'A Typesetter-independent TROFF', by Brian Kernighan, and by 1992 the
AT&T 'troff' manual was updated to incorprate a description of it.

   The GNU 'troff' intermediate output format is compatible with this
specification except for the following features.

   * The classical quasi-device independence is not yet implemented.

   * The old hardware was very different from what we use today.  So the
     'groff' devices are also fundamentally different from the ones in
     AT&T 'troff'.  For example, the AT&T PostScript device is called
     'post' and has a resolution of only 720 units per inch, suitable
     for printers 20 years ago, while 'groff''s 'ps' device has a
     resolution of 72000 units per inch.  Maybe, by implementing some
     rescaling mechanism similar to the classical quasi-device
     independence, 'groff' could emulate AT&T's 'post' device.

   * The B-spline command 'D~' is correctly handled by the intermediate
     output parser, but the drawing routines aren't implemented in some
     of the postprocessor programs.

   * The argument of the commands 's' and 'x H' has the implicit unit
     scaled point 'z' in 'gtroff', while AT&T 'troff' has point ('p').
     This isn't an incompatibility but a compatible extension, for both
     units coincide for all devices without a 'sizescale' parameter in
     the 'DESC' file, including all postprocessors from AT&T and
     'groff''s text devices.  The few 'groff' devices with a 'sizescale'
     parameter either do not exist for AT&T 'troff', have a different
     name, or seem to have a different resolution.  So conflicts are
     very unlikely.

   * The position changing after the commands 'Dp', 'DP', and 'Dt' is
     illogical, but as old versions of 'gtroff' used this feature it is
     kept for compatibility reasons.

6.2 Device and Font Description Files
=====================================

The 'groff' font and output device description formats are slight
extensions of those used by AT&T device-independent 'troff'.  In
distinction to the AT&T implementation, 'groff' lacks a binary format;
all files are text files.(1)  (*note Device and Font Description
Files-Footnote-1::) The device and font description files for a device
NAME are stored in a 'devNAME' directory.  The device description file
is called 'DESC', and, for each font supported by the device, a font
description file is called 'F', where F is usually an abbreviation of a
font's name and/or style.  For example, the 'ps' (PostScript) device has
'groff' font description files for Times roman ('TR') and Zapf Chancery
Medium italic ('ZCMI'), among many others, while the 'utf8' device (for
terminal emulators) has only font descriptions for the roman, italic,
bold, and bold-italic styles ('R', 'I', 'B', and 'BI', respectively).

   Device and font description files are read both by the formatter, GNU
'troff', and by output drivers.  The programs delegate these files'
processing to an internal library, 'libgroff', ensuring their consistent
interpretation.

   (1) Plan 9 'troff' has also abandoned the binary format.

6.2.1 'DESC' File Format
------------------------

The 'DESC' file contains a series of directives; each begins a line.
Their order is not important, with two exceptions: (1) the 'res'
directive must precede any 'papersize' directive; and (2) the 'charset'
directive must come last (if at all).  If a directive name is repeated,
later entries in the file override previous ones (except that the paper
dimensions are computed based on the 'res' directive last seen when
'papersize' is encountered).  Spaces and/or tabs separate words and are
ignored at line boundaries.  Comments start with the '#' character and
extend to the end of a line.  Empty lines are ignored.

'family FAM'
     The default font family is FAM.

'fonts N F1 ... FN'
     Fonts F1, ..., FN are mounted at font positions M+1, ..., M+N where
     M is the number of 'styles' (see below).  This directive may extend
     over more than one line.  A font name of '0' causes no font to be
     mounted at the corresponding position.

'hor N'
     The horizontal motion quantum is N basic units.  All horizontal
     quantities are rounded to multiples of N.

'image_generator PROGRAM'
     Use PROGRAM to generate PNG images from PostScript input.  Under
     GNU/Linux, this is usually 'gs', but under other systems (notably
     Cygwin) it might be set to another name.  The 'grohtml' driver uses
     this directive.

'paperlength N'
     The vertical dimension of the output medium is N basic units
     (deprecated: use 'papersize' instead).

'papersize FORMAT-OR-DIMENSION-PAIR-OR-FILE-NAME ...'
     The dimensions of the output medium are as according to the
     argument, which is either a standard paper format, a pair of
     dimensions, or the name of a plain text file containing either of
     the foregoing.

     Recognized paper formats are the ISO and DIN formats 'A0'-'A7',
     'B0'-'B7', 'C0'-'C7', 'D0'-'D7'; the U.S. paper types 'letter',
     'legal', 'tabloid', 'ledger', 'statement', and 'executive'; and the
     envelope formats 'com10', 'monarch', and 'DL'.  Matching is
     performed without regard for lettercase.

     Alternatively, the argument can be a custom paper format in the
     format 'LENGTH,WIDTH' (with no spaces before or after the comma).
     Both LENGTH and WIDTH must have a unit appended; valid units are
     'i' for inches, 'c' for centimeters, 'p' for points, and 'P' for
     picas.  Example: '12c,235p'.  An argument that starts with a digit
     is always treated as a custom paper format.

     Finally, the argument can be a file name (e.g., '/etc/papersize');
     if the file can be opened, the first line is read and a match
     attempted against each of the other forms.  No comment syntax is
     supported.

     More than one argument can be specified; each is scanned in turn
     and the first valid paper specification used.

'paperwidth N'
     The horizontal dimension of the output medium is N basic units
     (deprecated: use 'papersize' instead).

'pass_filenames'
     Direct GNU 'troff' to emit the name of the source file being
     processed.  This is achieved with the intermediate output command
     'x F', which 'grohtml' interprets.

'postpro PROGRAM'
     Use PROGRAM as the postprocessor.

'prepro PROGRAM'
     Use PROGRAM as a preprocessor.  The 'html' and 'xhtml' output
     devices use this directive.

'print PROGRAM'
     Use PROGRAM as a spooler program for printing.  If omitted, the
     '-l' and '-L' options of 'groff' are ignored.

'res N'
     The device resolution is N basic units per inch.

'sizes S1 ... SN 0'
     The device has fonts at S1, ..., SN scaled points (see below).  The
     list of sizes must be terminated by '0'.  Each SI can also be a
     range of sizes M-N.  The list can extend over more than one line.

'sizescale N'
     A typographical point is subdivided into N scaled points.  The
     default is '1'.  *Note Using Fractional Type Sizes::.

'styles S1 ... SM'
     The first M mounting positions are associated with styles S1, ...,
     SM.

'tcommand'
     The postprocessor can handle the 't' and 'u' intermediate output
     commands.

'unicode'
     The output device supports the complete Unicode repertoire.  This
     directive is useful only for devices that produce character
     entities instead of glyphs.

     If 'unicode' is present, no 'charset' section is required in the
     font description files since the Unicode handling built into
     'groff' is used.  However, if there are entries in a font
     description file's 'charset' section, they either override the
     default mappings for those particular characters or add new
     mappings (normally for composite characters).

     The 'utf8', 'html', and 'xhtml' output devices use this directive.

'unitwidth N'
     Quantities in the font description files are in basic units for
     fonts whose type size is N scaled points.

'unscaled_charwidths'
     Make the font handling module always return unscaled character
     widths.  The 'grohtml' driver uses this directive.

'use_charnames_in_special'
     GNU 'troff' should encode special characters inside device control
     commands; see *note Postprocessor Access::.  The 'grohtml' driver
     uses this directive.

'vert N'
     The vertical motion quantum is N basic units.  All vertical
     quantities are rounded to multiples of N.

'charset'
     This line and everything following it in the file are ignored.  It
     is recognized for compatibility with other 'troff' implementations.
     In GNU 'troff', character set repertoire is described on a per-font
     basis.

   GNU 'troff' recognizes but ignores the directives 'spare1', 'spare2',
and 'biggestfont'.

   The 'res', 'unitwidth', 'fonts', and 'sizes' lines are mandatory.
Directives not listed above are ignored by GNU 'troff' but may be used
by postprocessors to obtain further information about the device.

6.2.2 Font Description File Format
----------------------------------

On typesetting output devices, each font is typically available at
multiple sizes.  While paper measurements in the device description file
are in absolute units, measurements applicable to fonts must be
proportional to the type size.  'groff' achieves this using the
precedent set by AT&T device-independent 'troff': one font size is
chosen as a norm, and all others are scaled linearly relative to that
basis.  The "unit width" is the number of basic units per point when the
font is rendered at this nominal size.

   For instance, 'groff''s 'lbp' device uses a 'unitwidth' of 800.  Its
Times roman font 'TR' has a 'spacewidth' of 833; this is also the width
of its comma, period, centered period, and mathematical asterisk, while
its 'M' is 2,963 basic units.  Thus, an 'M' on the 'lbp' device is 2,963
basic units wide at a notional type size of 800 points.(1)  (*note Font
Description File Format-Footnote-1::)

   A font description file has two sections.  The first is a sequence of
directives, and is parsed similarly to the 'DESC' file described above.
Except for the directive names that begin the second section, their
ordering is immaterial.  Later directives of the same name override
earlier ones, spaces and tabs are handled in the same way, and the same
comment syntax is supported.  Empty lines are ignored throughout.

'name F'
     The name of the font is F.  'DESC' is an invalid font name.  Simple
     integers are valid, but their use is discouraged.(2)  (*note Font
     Description File Format-Footnote-2::)

'spacewidth N'
     The width of an unadjusted inter-word space is N basic units.

   The directives above must appear in the first section; those below
are optional.

'slant N'
     The font's glyphs have a slant of N degrees; a positive N slants in
     the direction of text flow.

'ligatures LIG1 ... LIGN [0]'
     Glyphs LIG1, ..., LIGN are ligatures; possible ligatures are 'ff',
     'fi', 'fl', 'ffi' and 'ffl'.  For compatibility with other 'troff'
     implementations, the list of ligatures may be terminated with
     a '0'.  The list of ligatures must not extend over more than one
     line.

'special'
     The font is "special": when a glyph is requested that is not
     present in the current font, it is sought in any mounted fonts that
     bear this property.

   Other directives in this section are ignored by GNU 'troff', but may
be used by postprocessors to obtain further information about the font.

   The second section contains one or two subsections.  These can appear
in either order; the first one encountered commences the second section.
Each starts with a directive on a line by itself.  A 'charset'
subsection is mandatory unless the associated 'DESC' file contains the
'unicode' directive.  Another subsection, 'kernpairs', is optional.

   The directive 'charset' starts the character set subsection.(3)
(*note Font Description File Format-Footnote-3::) It precedes a series
of glyph descriptions, one per line.  Each such glyph description
comprises a set of fields separated by spaces or tabs and organized as
follows.

     NAME METRICS TYPE CODE [ENTITY-NAME] ['--' COMMENT]

NAME identifies the glyph: if NAME is a printable character C, it
corresponds to the 'troff' ordinary character C.  If NAME is a
multi-character sequence not beginning with '\', it corresponds to the
GNU 'troff' special character escape sequence '\[NAME]'.  A name
consisting of three minus signs, '---', is special and indicates that
the glyph is unnamed: such glyphs can be accessed only by the '\N'
escape sequence in 'troff'.  A special character named '---' can still
be defined using 'char' and similar requests.  The NAME '\-' defines the
minus sign glyph.  Finally, NAME can be the unbreakable one-sixth and
one-twelfth space escape sequences, '\|' and '\^' ("thin" and "hair"
spaces, respectively), in which case only the width metric described
below is interpreted; a font can thus customize the widths of these
spaces.

   The form of the METRICS field is as follows.

     WIDTH[','[HEIGHT[','[DEPTH[','[ITALIC-CORRECTION
       [','[LEFT-ITALIC-CORRECTION[','[SUBSCRIPT-CORRECTION]]]]]]]]]]

There must not be any spaces, tabs, or newlines between these
"subfields" (which have been split here into two lines only for better
legibility).  The subfields are in basic units expressed as decimal
integers.  Unspecified subfields default to '0'.  Since there is no
associated binary format, these values are not required to fit into the
C language data type 'char' as they are in AT&T device-independent
'troff'.

   The WIDTH subfield gives the width of the glyph.  The HEIGHT subfield
gives the height of the glyph (upward is positive); if a glyph does not
extend above the baseline, it should be given a zero height, rather than
a negative height.  The DEPTH subfield gives the depth of the glyph,
that is, the distance below the baseline to which the glyph extends
(downward is positive); if a glyph does not extend below the baseline,
it should be given a zero depth, rather than a negative depth.  Italic
corrections are relevant to glyphs in italic or oblique styles.  The
ITALIC-CORRECTION is the amount of space that should be added after an
oblique glyph to be followed immediately by an upright glyph.  The
LEFT-ITALIC-CORRECTION is the amount of space that should be added
before an oblique glyph to be preceded immediately by an upright glyph.
The SUBSCRIPT-CORRECTION is the amount of space that should be added
after an oblique glyph to be followed by a subscript; it should be less
than the italic correction.

   For fonts used with typesetting devices, the TYPE field gives a
featural description of the glyph: it is a bit mask recording whether
the glyph is an ascender, descender, both, or neither.  When a '\w'
escape sequence is interpolated, these values are bitwise or-ed together
for each glyph and stored in the 'nr' register.  In font descriptions
for terminal devices, all glyphs might have a type of zero, regardless
of their appearance.

'0'
     means the glyph lies entirely between the baseline and a horizontal
     line at the "x-height" of the font; typical examples are 'a', 'c',
     and 'x';

'1'
     means the glyph descends below the baseline, like 'p';

'2'
     means the glyph ascends above the font's x-height, like 'A' or 'b';
     and

'3'
     means the glyph is both an ascender and a descender--this is true
     of parentheses in some fonts.

   The CODE field gives a numeric identifier that the postprocessor uses
to render the glyph.  The glyph can be specified to 'troff' using this
code by means of the '\N' escape sequence.  CODE can be any integer.(4)
(*note Font Description File Format-Footnote-4::)

   The ENTITY-NAME field defines an identifier for the glyph that the
postprocessor uses to print the GNU 'troff' glyph NAME.  This field is
optional; it was introduced so that the 'grohtml' output driver could
encode its character set.  For example, the glyph '\[Po]' is represented
by '&pound;' in HTML 4.0.  For efficiency, these data are now compiled
directly into 'grohtml'.  'grops' uses the field to build sub-encoding
arrays for PostScript fonts containing more than 256 glyphs.  Anything
on the line after the ENTITY-NAME field or '--' is ignored.

   A line in the 'charset' section can also have the form

     NAME "

identifying NAME as another name for the glyph mentioned in the
preceding line.  Such aliases can be chained.

   The directive 'kernpairs' starts a list of kerning adjustments to be
made to adjacent glyph pairs from this font.  It contains a sequence of
lines formatted as follows.

     G1 G2 N

The foregoing means that when glyph G1 is typeset immediately before G2,
the space between them should be increased by N.  Most kerning pairs
should have a negative value for N.

   (1) 800-point type is not practical for most purposes, but using it
enables the quantities in the font description files to be expressed as
integers.

   (2) 'groff' requests and escape sequences interpret non-negative font
names as mounting positions instead.  Further, a font named '0' cannot
be automatically mounted by the 'fonts' directive of a 'DESC' file.

   (3) For typesetter devices, this directive is misnamed since it
starts a list of glyphs, not characters.

   (4) that is, any integer parsable by the C standard library's
'strtol(3)' function

Appendix A Copying This Manual
******************************

                     Version 1.3, 3 November 2008

     Copyright � 2000-2018 Free Software Foundation, Inc.
     <http://fsf.org/>

     Everyone is permitted to copy and distribute verbatim copies
     of this license document, but changing it is not allowed.

  0. PREAMBLE

     The purpose of this License is to make a manual, textbook, or other
     functional and useful document "free" in the sense of freedom: to
     assure everyone the effective freedom to copy and redistribute it,
     with or without modifying it, either commercially or
     noncommercially.  Secondarily, this License preserves for the
     author and publisher a way to get credit for their work, while not
     being considered responsible for modifications made by others.

     This License is a kind of "copyleft", which means that derivative
     works of the document must themselves be free in the same sense.
     It complements the GNU General Public License, which is a copyleft
     license designed for free software.

     We have designed this License in order to use it for manuals for
     free software, because free software needs free documentation: a
     free program should come with manuals providing the same freedoms
     that the software does.  But this License is not limited to
     software manuals; it can be used for any textual work, regardless
     of subject matter or whether it is published as a printed book.  We
     recommend this License principally for works whose purpose is
     instruction or reference.

  1. APPLICABILITY AND DEFINITIONS

     This License applies to any manual or other work, in any medium,
     that contains a notice placed by the copyright holder saying it can
     be distributed under the terms of this License.  Such a notice
     grants a world-wide, royalty-free license, unlimited in duration,
     to use that work under the conditions stated herein.  The
     "Document", below, refers to any such manual or work.  Any member
     of the public is a licensee, and is addressed as "you".  You accept
     the license if you copy, modify or distribute the work in a way
     requiring permission under copyright law.

     A "Modified Version" of the Document means any work containing the
     Document or a portion of it, either copied verbatim, or with
     modifications and/or translated into another language.

     A "Secondary Section" is a named appendix or a front-matter section
     of the Document that deals exclusively with the relationship of the
     publishers or authors of the Document to the Document's overall
     subject (or to related matters) and contains nothing that could
     fall directly within that overall subject.  (Thus, if the Document
     is in part a textbook of mathematics, a Secondary Section may not
     explain any mathematics.)  The relationship could be a matter of
     historical connection with the subject or with related matters, or
     of legal, commercial, philosophical, ethical or political position
     regarding them.

     The "Invariant Sections" are certain Secondary Sections whose
     titles are designated, as being those of Invariant Sections, in the
     notice that says that the Document is released under this License.
     If a section does not fit the above definition of Secondary then it
     is not allowed to be designated as Invariant.  The Document may
     contain zero Invariant Sections.  If the Document does not identify
     any Invariant Sections then there are none.

     The "Cover Texts" are certain short passages of text that are
     listed, as Front-Cover Texts or Back-Cover Texts, in the notice
     that says that the Document is released under this License.  A
     Front-Cover Text may be at most 5 words, and a Back-Cover Text may
     be at most 25 words.

     A "Transparent" copy of the Document means a machine-readable copy,
     represented in a format whose specification is available to the
     general public, that is suitable for revising the document
     straightforwardly with generic text editors or (for images composed
     of pixels) generic paint programs or (for drawings) some widely
     available drawing editor, and that is suitable for input to text
     formatters or for automatic translation to a variety of formats
     suitable for input to text formatters.  A copy made in an otherwise
     Transparent file format whose markup, or absence of markup, has
     been arranged to thwart or discourage subsequent modification by
     readers is not Transparent.  An image format is not Transparent if
     used for any substantial amount of text.  A copy that is not
     "Transparent" is called "Opaque".

     Examples of suitable formats for Transparent copies include plain
     ASCII without markup, Texinfo input format, LaTeX input format,
     SGML or XML using a publicly available DTD, and standard-conforming
     simple HTML, PostScript or PDF designed for human modification.
     Examples of transparent image formats include PNG, XCF and JPG.
     Opaque formats include proprietary formats that can be read and
     edited only by proprietary word processors, SGML or XML for which
     the DTD and/or processing tools are not generally available, and
     the machine-generated HTML, PostScript or PDF produced by some word
     processors for output purposes only.

     The "Title Page" means, for a printed book, the title page itself,
     plus such following pages as are needed to hold, legibly, the
     material this License requires to appear in the title page.  For
     works in formats which do not have any title page as such, "Title
     Page" means the text near the most prominent appearance of the
     work's title, preceding the beginning of the body of the text.

     The "publisher" means any person or entity that distributes copies
     of the Document to the public.

     A section "Entitled XYZ" means a named subunit of the Document
     whose title either is precisely XYZ or contains XYZ in parentheses
     following text that translates XYZ in another language.  (Here XYZ
     stands for a specific section name mentioned below, such as
     "Acknowledgements", "Dedications", "Endorsements", or "History".)
     To "Preserve the Title" of such a section when you modify the
     Document means that it remains a section "Entitled XYZ" according
     to this definition.

     The Document may include Warranty Disclaimers next to the notice
     which states that this License applies to the Document.  These
     Warranty Disclaimers are considered to be included by reference in
     this License, but only as regards disclaiming warranties: any other
     implication that these Warranty Disclaimers may have is void and
     has no effect on the meaning of this License.

  2. VERBATIM COPYING

     You may copy and distribute the Document in any medium, either
     commercially or noncommercially, provided that this License, the
     copyright notices, and the license notice saying this License
     applies to the Document are reproduced in all copies, and that you
     add no other conditions whatsoever to those of this License.  You
     may not use technical measures to obstruct or control the reading
     or further copying of the copies you make or distribute.  However,
     you may accept compensation in exchange for copies.  If you
     distribute a large enough number of copies you must also follow the
     conditions in section 3.

     You may also lend copies, under the same conditions stated above,
     and you may publicly display copies.

  3. COPYING IN QUANTITY

     If you publish printed copies (or copies in media that commonly
     have printed covers) of the Document, numbering more than 100, and
     the Document's license notice requires Cover Texts, you must
     enclose the copies in covers that carry, clearly and legibly, all
     these Cover Texts: Front-Cover Texts on the front cover, and
     Back-Cover Texts on the back cover.  Both covers must also clearly
     and legibly identify you as the publisher of these copies.  The
     front cover must present the full title with all words of the title
     equally prominent and visible.  You may add other material on the
     covers in addition.  Copying with changes limited to the covers, as
     long as they preserve the title of the Document and satisfy these
     conditions, can be treated as verbatim copying in other respects.

     If the required texts for either cover are too voluminous to fit
     legibly, you should put the first ones listed (as many as fit
     reasonably) on the actual cover, and continue the rest onto
     adjacent pages.

     If you publish or distribute Opaque copies of the Document
     numbering more than 100, you must either include a machine-readable
     Transparent copy along with each Opaque copy, or state in or with
     each Opaque copy a computer-network location from which the general
     network-using public has access to download using public-standard
     network protocols a complete Transparent copy of the Document, free
     of added material.  If you use the latter option, you must take
     reasonably prudent steps, when you begin distribution of Opaque
     copies in quantity, to ensure that this Transparent copy will
     remain thus accessible at the stated location until at least one
     year after the last time you distribute an Opaque copy (directly or
     through your agents or retailers) of that edition to the public.

     It is requested, but not required, that you contact the authors of
     the Document well before redistributing any large number of copies,
     to give them a chance to provide you with an updated version of the
     Document.

  4. MODIFICATIONS

     You may copy and distribute a Modified Version of the Document
     under the conditions of sections 2 and 3 above, provided that you
     release the Modified Version under precisely this License, with the
     Modified Version filling the role of the Document, thus licensing
     distribution and modification of the Modified Version to whoever
     possesses a copy of it.  In addition, you must do these things in
     the Modified Version:

       A. Use in the Title Page (and on the covers, if any) a title
          distinct from that of the Document, and from those of previous
          versions (which should, if there were any, be listed in the
          History section of the Document).  You may use the same title
          as a previous version if the original publisher of that
          version gives permission.

       B. List on the Title Page, as authors, one or more persons or
          entities responsible for authorship of the modifications in
          the Modified Version, together with at least five of the
          principal authors of the Document (all of its principal
          authors, if it has fewer than five), unless they release you
          from this requirement.

       C. State on the Title page the name of the publisher of the
          Modified Version, as the publisher.

       D. Preserve all the copyright notices of the Document.

       E. Add an appropriate copyright notice for your modifications
          adjacent to the other copyright notices.

       F. Include, immediately after the copyright notices, a license
          notice giving the public permission to use the Modified
          Version under the terms of this License, in the form shown in
          the Addendum below.

       G. Preserve in that license notice the full lists of Invariant
          Sections and required Cover Texts given in the Document's
          license notice.

       H. Include an unaltered copy of this License.

       I. Preserve the section Entitled "History", Preserve its Title,
          and add to it an item stating at least the title, year, new
          authors, and publisher of the Modified Version as given on the
          Title Page.  If there is no section Entitled "History" in the
          Document, create one stating the title, year, authors, and
          publisher of the Document as given on its Title Page, then add
          an item describing the Modified Version as stated in the
          previous sentence.

       J. Preserve the network location, if any, given in the Document
          for public access to a Transparent copy of the Document, and
          likewise the network locations given in the Document for
          previous versions it was based on.  These may be placed in the
          "History" section.  You may omit a network location for a work
          that was published at least four years before the Document
          itself, or if the original publisher of the version it refers
          to gives permission.

       K. For any section Entitled "Acknowledgements" or "Dedications",
          Preserve the Title of the section, and preserve in the section
          all the substance and tone of each of the contributor
          acknowledgements and/or dedications given therein.

       L. Preserve all the Invariant Sections of the Document, unaltered
          in their text and in their titles.  Section numbers or the
          equivalent are not considered part of the section titles.

       M. Delete any section Entitled "Endorsements".  Such a section
          may not be included in the Modified Version.

       N. Do not retitle any existing section to be Entitled
          "Endorsements" or to conflict in title with any Invariant
          Section.

       O. Preserve any Warranty Disclaimers.

     If the Modified Version includes new front-matter sections or
     appendices that qualify as Secondary Sections and contain no
     material copied from the Document, you may at your option designate
     some or all of these sections as invariant.  To do this, add their
     titles to the list of Invariant Sections in the Modified Version's
     license notice.  These titles must be distinct from any other
     section titles.

     You may add a section Entitled "Endorsements", provided it contains
     nothing but endorsements of your Modified Version by various
     parties--for example, statements of peer review or that the text
     has been approved by an organization as the authoritative
     definition of a standard.

     You may add a passage of up to five words as a Front-Cover Text,
     and a passage of up to 25 words as a Back-Cover Text, to the end of
     the list of Cover Texts in the Modified Version.  Only one passage
     of Front-Cover Text and one of Back-Cover Text may be added by (or
     through arrangements made by) any one entity.  If the Document
     already includes a cover text for the same cover, previously added
     by you or by arrangement made by the same entity you are acting on
     behalf of, you may not add another; but you may replace the old
     one, on explicit permission from the previous publisher that added
     the old one.

     The author(s) and publisher(s) of the Document do not by this
     License give permission to use their names for publicity for or to
     assert or imply endorsement of any Modified Version.

  5. COMBINING DOCUMENTS

     You may combine the Document with other documents released under
     this License, under the terms defined in section 4 above for
     modified versions, provided that you include in the combination all
     of the Invariant Sections of all of the original documents,
     unmodified, and list them all as Invariant Sections of your
     combined work in its license notice, and that you preserve all
     their Warranty Disclaimers.

     The combined work need only contain one copy of this License, and
     multiple identical Invariant Sections may be replaced with a single
     copy.  If there are multiple Invariant Sections with the same name
     but different contents, make the title of each such section unique
     by adding at the end of it, in parentheses, the name of the
     original author or publisher of that section if known, or else a
     unique number.  Make the same adjustment to the section titles in
     the list of Invariant Sections in the license notice of the
     combined work.

     In the combination, you must combine any sections Entitled
     "History" in the various original documents, forming one section
     Entitled "History"; likewise combine any sections Entitled
     "Acknowledgements", and any sections Entitled "Dedications".  You
     must delete all sections Entitled "Endorsements."

  6. COLLECTIONS OF DOCUMENTS

     You may make a collection consisting of the Document and other
     documents released under this License, and replace the individual
     copies of this License in the various documents with a single copy
     that is included in the collection, provided that you follow the
     rules of this License for verbatim copying of each of the documents
     in all other respects.

     You may extract a single document from such a collection, and
     distribute it individually under this License, provided you insert
     a copy of this License into the extracted document, and follow this
     License in all other respects regarding verbatim copying of that
     document.

  7. AGGREGATION WITH INDEPENDENT WORKS

     A compilation of the Document or its derivatives with other
     separate and independent documents or works, in or on a volume of a
     storage or distribution medium, is called an "aggregate" if the
     copyright resulting from the compilation is not used to limit the
     legal rights of the compilation's users beyond what the individual
     works permit.  When the Document is included in an aggregate, this
     License does not apply to the other works in the aggregate which
     are not themselves derivative works of the Document.

     If the Cover Text requirement of section 3 is applicable to these
     copies of the Document, then if the Document is less than one half
     of the entire aggregate, the Document's Cover Texts may be placed
     on covers that bracket the Document within the aggregate, or the
     electronic equivalent of covers if the Document is in electronic
     form.  Otherwise they must appear on printed covers that bracket
     the whole aggregate.

  8. TRANSLATION

     Translation is considered a kind of modification, so you may
     distribute translations of the Document under the terms of section
     4.  Replacing Invariant Sections with translations requires special
     permission from their copyright holders, but you may include
     translations of some or all Invariant Sections in addition to the
     original versions of these Invariant Sections.  You may include a
     translation of this License, and all the license notices in the
     Document, and any Warranty Disclaimers, provided that you also
     include the original English version of this License and the
     original versions of those notices and disclaimers.  In case of a
     disagreement between the translation and the original version of
     this License or a notice or disclaimer, the original version will
     prevail.

     If a section in the Document is Entitled "Acknowledgements",
     "Dedications", or "History", the requirement (section 4) to
     Preserve its Title (section 1) will typically require changing the
     actual title.

  9. TERMINATION

     You may not copy, modify, sublicense, or distribute the Document
     except as expressly provided under this License.  Any attempt
     otherwise to copy, modify, sublicense, or distribute it is void,
     and will automatically terminate your rights under this License.

     However, if you cease all violation of this License, then your
     license from a particular copyright holder is reinstated (a)
     provisionally, unless and until the copyright holder explicitly and
     finally terminates your license, and (b) permanently, if the
     copyright holder fails to notify you of the violation by some
     reasonable means prior to 60 days after the cessation.

     Moreover, your license from a particular copyright holder is
     reinstated permanently if the copyright holder notifies you of the
     violation by some reasonable means, this is the first time you have
     received notice of violation of this License (for any work) from
     that copyright holder, and you cure the violation prior to 30 days
     after your receipt of the notice.

     Termination of your rights under this section does not terminate
     the licenses of parties who have received copies or rights from you
     under this License.  If your rights have been terminated and not
     permanently reinstated, receipt of a copy of some or all of the
     same material does not give you any rights to use it.

  10. FUTURE REVISIONS OF THIS LICENSE

     The Free Software Foundation may publish new, revised versions of
     the GNU Free Documentation License from time to time.  Such new
     versions will be similar in spirit to the present version, but may
     differ in detail to address new problems or concerns.  See
     <http://www.gnu.org/copyleft/>.

     Each version of the License is given a distinguishing version
     number.  If the Document specifies that a particular numbered
     version of this License "or any later version" applies to it, you
     have the option of following the terms and conditions either of
     that specified version or of any later version that has been
     published (not as a draft) by the Free Software Foundation.  If the
     Document does not specify a version number of this License, you may
     choose any version ever published (not as a draft) by the Free
     Software Foundation.  If the Document specifies that a proxy can
     decide which future versions of this License can be used, that
     proxy's public statement of acceptance of a version permanently
     authorizes you to choose that version for the Document.

  11. RELICENSING

     "Massive Multiauthor Collaboration Site" (or "MMC Site") means any
     World Wide Web server that publishes copyrightable works and also
     provides prominent facilities for anybody to edit those works.  A
     public wiki that anybody can edit is an example of such a server.
     A "Massive Multiauthor Collaboration" (or "MMC") contained in the
     site means any set of copyrightable works thus published on the MMC
     site.

     "CC-BY-SA" means the Creative Commons Attribution-Share Alike 3.0
     license published by Creative Commons Corporation, a not-for-profit
     corporation with a principal place of business in San Francisco,
     California, as well as future copyleft versions of that license
     published by that same organization.

     "Incorporate" means to publish or republish a Document, in whole or
     in part, as part of another Document.

     An MMC is "eligible for relicensing" if it is licensed under this
     License, and if all works that were first published under this
     License somewhere other than this MMC, and subsequently
     incorporated in whole or in part into the MMC, (1) had no cover
     texts or invariant sections, and (2) were thus incorporated prior
     to November 1, 2008.

     The operator of an MMC Site may republish an MMC contained in the
     site under CC-BY-SA on the same site at any time before August 1,
     2009, provided the MMC is eligible for relicensing.

ADDENDUM: How to use this License for your documents
====================================================

To use this License in a document you have written, include a copy of
the License in the document and put the following copyright and license
notices just after the title page:

       Copyright (C)  YEAR  YOUR NAME.
       Permission is granted to copy, distribute and/or modify this document
       under the terms of the GNU Free Documentation License, Version 1.3
       or any later version published by the Free Software Foundation;
       with no Invariant Sections, no Front-Cover Texts, and no Back-Cover
       Texts.  A copy of the license is included in the section entitled ``GNU
       Free Documentation License''.

   If you have Invariant Sections, Front-Cover Texts and Back-Cover
Texts, replace the "with...Texts." line with this:

         with the Invariant Sections being LIST THEIR TITLES, with
         the Front-Cover Texts being LIST, and with the Back-Cover Texts
         being LIST.

   If you have Invariant Sections without Cover Texts, or some other
combination of the three, merge those two alternatives to suit the
situation.

   If your document contains nontrivial examples of program code, we
recommend releasing these examples in parallel under your choice of free
software license, such as the GNU General Public License, to permit
their use in free software.

Appendix B Request Index
************************

Request names appear without a leading control character; the defaults
are '.' for the regular control character and ''' for the no-break
control character.

* Menu:

* ab:                                    Debugging.         (line 12127)
* ad:                                    Manipulating Filling and Adjustment.
                                                            (line  5701)
* af:                                    Assigning Register Formats.
                                                            (line  5419)
* aln:                                   Setting Registers. (line  5323)
* als:                                   Strings.           (line  8972)
* am:                                    Writing Macros.    (line  9530)
* am1:                                   Writing Macros.    (line  9531)
* ami:                                   Writing Macros.    (line  9532)
* ami1:                                  Writing Macros.    (line  9533)
* as:                                    Strings.           (line  8888)
* as1:                                   Strings.           (line  8889)
* asciify:                               Diversions.        (line 11120)
* backtrace:                             Debugging.         (line 12176)
* bd:                                    Artificial Fonts.  (line  8211)
* blm:                                   Blank Line Traps.  (line 10779)
* box:                                   Diversions.        (line 10975)
* boxa:                                  Diversions.        (line 10976)
* bp:                                    Page Control.      (line  7175)
* br:                                    Manipulating Filling and Adjustment.
                                                            (line  5657)
* break:                                 while.             (line  9393)
* brp:                                   Manipulating Filling and Adjustment.
                                                            (line  5774)
* c2:                                    Control Characters.
                                                            (line  4749)
* cc:                                    Control Characters.
                                                            (line  4743)
* ce:                                    Manipulating Filling and Adjustment.
                                                            (line  5826)
* cf:                                    I/O.               (line 11521)
* cflags:                                Using Symbols.     (line  7846)
* ch:                                    Page Location Traps.
                                                            (line 10528)
* char:                                  Using Symbols.     (line  7945)
* chop:                                  Strings.           (line  8919)
* class:                                 Character Classes. (line  8034)
* close:                                 I/O.               (line 11703)
* color:                                 Colors.            (line  8687)
* composite:                             Using Symbols.     (line  7802)
* continue:                              while.             (line  9397)
* cp:                                    Compatibility Mode.
                                                            (line 12414)
* cs:                                    Artificial Fonts.  (line  8241)
* cu:                                    Artificial Fonts.  (line  8202)
* da:                                    Diversions.        (line 10944)
* de:                                    Writing Macros.    (line  9418)
* de1:                                   Writing Macros.    (line  9490)
* defcolor:                              Colors.            (line  8699)
* dei:                                   Writing Macros.    (line  9512)
* dei1:                                  Writing Macros.    (line  9513)
* device:                                Postprocessor Access.
                                                            (line 11738)
* devicem:                               Postprocessor Access.
                                                            (line 11768)
* di:                                    Diversions.        (line 10943)
* do:                                    Compatibility Mode.
                                                            (line 12425)
* ds:                                    ms Document Control Settings.
                                                            (line  1803)
* ds <1>:                                Strings.           (line  8798)
* ds1:                                   Strings.           (line  8799)
* dt:                                    Diversion Traps.   (line 10668)
* ec:                                    Using Escape Sequences.
                                                            (line  5012)
* ecr:                                   Using Escape Sequences.
                                                            (line  5038)
* ecs:                                   Using Escape Sequences.
                                                            (line  5037)
* el:                                    if-else.           (line  9213)
* em:                                    End-of-input Traps.
                                                            (line 10807)
* eo:                                    Using Escape Sequences.
                                                            (line  5007)
* ev:                                    Environments.      (line 11308)
* evc:                                   Environments.      (line 11362)
* ex:                                    Debugging.         (line 12132)
* fam:                                   Font Families.     (line  7463)
* fc:                                    Fields.            (line  6704)
* fchar:                                 Using Symbols.     (line  7946)
* fcolor:                                Colors.            (line  8757)
* fi:                                    Manipulating Filling and Adjustment.
                                                            (line  5684)
* fl:                                    Debugging.         (line 12167)
* fp:                                    Font Positions.    (line  7550)
* fschar:                                Using Symbols.     (line  7947)
* fspecial:                              Special Fonts.     (line  8104)
* ft:                                    Selecting Fonts.   (line  7348)
* ftr:                                   Selecting Fonts.   (line  7406)
* fzoom:                                 Selecting Fonts.   (line  7420)
* gcolor:                                Colors.            (line  8729)
* hc:                                    Manipulating Hyphenation.
                                                            (line  6023)
* hcode:                                 Manipulating Hyphenation.
                                                            (line  6228)
* hla:                                   Manipulating Hyphenation.
                                                            (line  6262)
* hlm:                                   Manipulating Hyphenation.
                                                            (line  6275)
* hpf:                                   Manipulating Hyphenation.
                                                            (line  6168)
* hpfa:                                  Manipulating Hyphenation.
                                                            (line  6169)
* hpfcode:                               Manipulating Hyphenation.
                                                            (line  6170)
* hw:                                    Manipulating Hyphenation.
                                                            (line  5957)
* hy:                                    Manipulating Hyphenation.
                                                            (line  6055)
* hym:                                   Manipulating Hyphenation.
                                                            (line  6289)
* hys:                                   Manipulating Hyphenation.
                                                            (line  6304)
* ie:                                    if-else.           (line  9212)
* if:                                    if-then.           (line  9171)
* ig:                                    Comments.          (line  5180)
* in:                                    Line Layout.       (line  6935)
* it:                                    Input Line Traps.  (line 10679)
* itc:                                   Input Line Traps.  (line 10680)
* kern:                                  Ligatures and Kerning.
                                                            (line  8288)
* lc:                                    Leaders.           (line  6661)
* length:                                Strings.           (line  8909)
* lf:                                    Debugging.         (line 12103)
* lg:                                    Ligatures and Kerning.
                                                            (line  8270)
* linetabs:                              Tabs and Fields.   (line  6614)
* ll:                                    Line Layout.       (line  6987)
* ls:                                    Manipulating Spacing.
                                                            (line  6395)
* lsm:                                   Leading Space Traps.
                                                            (line 10789)
* lt:                                    Page Layout.       (line  7134)
* mc:                                    Miscellaneous.     (line 11897)
* mk:                                    Page Motions.      (line  9867)
* mso:                                   I/O.               (line 11512)
* msoquiet:                              I/O.               (line 11513)
* na:                                    Manipulating Filling and Adjustment.
                                                            (line  5768)
* ne:                                    Page Control.      (line  7195)
* nf:                                    Manipulating Filling and Adjustment.
                                                            (line  5692)
* nh:                                    Manipulating Hyphenation.
                                                            (line  6163)
* nm:                                    Miscellaneous.     (line 11796)
* nn:                                    Miscellaneous.     (line 11861)
* nop:                                   if-then.           (line  9190)
* nr:                                    ms Document Control Settings.
                                                            (line  1799)
* nr <1>:                                Setting Registers. (line  5223)
* nr <2>:                                Setting Registers. (line  5277)
* nr <3>:                                Auto-increment.    (line  5372)
* nroff:                                 troff and nroff Modes.
                                                            (line  6840)
* ns:                                    Manipulating Spacing.
                                                            (line  6454)
* nx:                                    I/O.               (line 11553)
* open:                                  I/O.               (line 11670)
* opena:                                 I/O.               (line 11671)
* os:                                    Page Control.      (line  7230)
* output:                                Diversions.        (line 11107)
* pc:                                    Page Layout.       (line  7149)
* pev:                                   Debugging.         (line 12150)
* pi:                                    I/O.               (line 11612)
* pl:                                    Page Layout.       (line  7090)
* pm:                                    Debugging.         (line 12154)
* pn:                                    Page Layout.       (line  7104)
* pnr:                                   Debugging.         (line 12158)
* po:                                    Line Layout.       (line  6909)
* ps:                                    Changing the Type Size.
                                                            (line  8462)
* psbb:                                  Miscellaneous.     (line 11945)
* pso:                                   I/O.               (line 11501)
* ptr:                                   Debugging.         (line 12162)
* pvs:                                   Changing the Vertical Spacing.
                                                            (line  8581)
* rchar:                                 Using Symbols.     (line  8004)
* rd:                                    I/O.               (line 11558)
* return:                                Writing Macros.    (line  9567)
* rfschar:                               Using Symbols.     (line  8005)
* rj:                                    Manipulating Filling and Adjustment.
                                                            (line  5865)
* rm:                                    Strings.           (line  8967)
* rn:                                    Strings.           (line  8964)
* rnn:                                   Setting Registers. (line  5318)
* rr:                                    Setting Registers. (line  5312)
* rs:                                    Manipulating Spacing.
                                                            (line  6455)
* rt:                                    Page Motions.      (line  9868)
* schar:                                 Using Symbols.     (line  7948)
* shc:                                   Manipulating Hyphenation.
                                                            (line  6032)
* shift:                                 Parameters.        (line  9610)
* sizes:                                 Changing the Type Size.
                                                            (line  8526)
* so:                                    I/O.               (line 11472)
* soquiet:                               I/O.               (line 11473)
* sp:                                    Manipulating Spacing.
                                                            (line  6348)
* special:                               Special Fonts.     (line  8103)
* spreadwarn:                            Debugging.         (line 12207)
* ss:                                    Manipulating Filling and Adjustment.
                                                            (line  5885)
* stringdown:                            Strings.           (line  8944)
* stringup:                              Strings.           (line  8945)
* sty:                                   Font Families.     (line  7504)
* substring:                             Strings.           (line  8927)
* sv:                                    Page Control.      (line  7229)
* sy:                                    I/O.               (line 11634)
* ta:                                    Tabs and Fields.   (line  6488)
* tag:                                   Postprocessor Access.
                                                            (line 11781)
* taga:                                  Postprocessor Access.
                                                            (line 11782)
* tc:                                    Tabs and Fields.   (line  6602)
* ti:                                    Line Layout.       (line  6959)
* tkf:                                   Ligatures and Kerning.
                                                            (line  8307)
* tl:                                    Page Layout.       (line  7120)
* tm:                                    Debugging.         (line 12115)
* tm1:                                   Debugging.         (line 12116)
* tmc:                                   Debugging.         (line 12117)
* tr:                                    Character Translations.
                                                            (line  6729)
* trf:                                   I/O.               (line 11520)
* trin:                                  Character Translations.
                                                            (line  6730)
* trnt:                                  Character Translations.
                                                            (line  6795)
* troff:                                 troff and nroff Modes.
                                                            (line  6832)
* uf:                                    Artificial Fonts.  (line  8206)
* ul:                                    Artificial Fonts.  (line  8180)
* unformat:                              Diversions.        (line 11145)
* vpt:                                   Vertical Position Traps.
                                                            (line 10406)
* vs:                                    Changing the Vertical Spacing.
                                                            (line  8540)
* warn:                                  Debugging.         (line 12226)
* warnscale:                             Debugging.         (line 12203)
* wh:                                    Page Location Traps.
                                                            (line 10425)
* while:                                 while.             (line  9331)
* write:                                 I/O.               (line 11682)
* writec:                                I/O.               (line 11683)
* writem:                                I/O.               (line 11694)

Appendix C Escape Sequence Index
********************************

The escape character, '\' by default, is always followed by at least one
more input character, making an escape _sequence_.  Any input token '\X'
with X not in the list below emits a warning and interpolates glyph X.
Note the entries for '\.', which may be obscured by the leader dots, and
for '\<RET>' and '\<SP>', which are sorted alphabetically, not by code
point order.

* Menu:

* \:                                     Using Escape Sequences.
                                                            (line  4944)
* \ <1>:                                 Using Symbols.     (line  7731)
* \!:                                    Diversions.        (line 11065)
* \":                                    Comments.          (line  5136)
* \#:                                    Comments.          (line  5170)
* \$:                                    Parameters.        (line  9602)
* \$*:                                   Parameters.        (line  9622)
* \$0:                                   Parameters.        (line  9657)
* \$@:                                   Parameters.        (line  9623)
* \$^:                                   Parameters.        (line  9624)
* \%:                                    Manipulating Hyphenation.
                                                            (line  5991)
* \&:                                    Dummy Characters.  (line  8357)
* \':                                    Using Symbols.     (line  7831)
* \(:                                    Using Symbols.     (line  7733)
* \):                                    Dummy Characters.  (line  8407)
* \*:                                    Strings.           (line  8800)
* \,:                                    Italic Corrections.
                                                            (line  8338)
* \-:                                    Using Symbols.     (line  7838)
* \.:                                    Copy Mode.         (line  9739)
* \/:                                    Italic Corrections.
                                                            (line  8328)
* \0:                                    Page Motions.      (line 10014)
* \::                                    Manipulating Hyphenation.
                                                            (line  5992)
* \?:                                    Diversions.        (line 11066)
* \A:                                    Identifiers.       (line  4628)
* \a:                                    Leaders.           (line  6658)
* \B:                                    Numeric Expressions.
                                                            (line  4522)
* \b:                                    Drawing Geometric Objects.
                                                            (line 10308)
* \c:                                    Line Continuation. (line  7047)
* \C:                                    Using Symbols.     (line  7793)
* \d:                                    Page Motions.      (line  9965)
* \D:                                    Drawing Geometric Objects.
                                                            (line 10216)
* \e:                                    Using Escape Sequences.
                                                            (line  5000)
* \E:                                    Copy Mode.         (line  9794)
* \f:                                    Selecting Fonts.   (line  7349)
* \F:                                    Font Families.     (line  7465)
* \g:                                    Assigning Register Formats.
                                                            (line  5477)
* \H:                                    Artificial Fonts.  (line  8129)
* \h:                                    Page Motions.      (line  9982)
* \k:                                    Page Motions.      (line 10082)
* \l:                                    Drawing Geometric Objects.
                                                            (line 10141)
* \L:                                    Drawing Geometric Objects.
                                                            (line 10167)
* \m:                                    Colors.            (line  8730)
* \M:                                    Colors.            (line  8758)
* \n:                                    Interpolating Registers.
                                                            (line  5338)
* \n <1>:                                Auto-increment.    (line  5381)
* \N:                                    Using Symbols.     (line  7810)
* \newline:                              Line Continuation. (line  7019)
* \o:                                    Page Motions.      (line 10097)
* \O:                                    Suppressing Output.
                                                            (line 11410)
* \p:                                    Manipulating Filling and Adjustment.
                                                            (line  5775)
* \R:                                    Setting Registers. (line  5224)
* \R <1>:                                Setting Registers. (line  5279)
* \r:                                    Page Motions.      (line  9963)
* \<RET>:                                Line Continuation. (line  7019)
* \S:                                    Artificial Fonts.  (line  8160)
* \s:                                    Changing the Type Size.
                                                            (line  8465)
* \<SP>:                                 Page Motions.      (line  9998)
* \space:                                Page Motions.      (line  9998)
* \t:                                    Tabs and Fields.   (line  6485)
* \u:                                    Page Motions.      (line  9964)
* \v:                                    Page Motions.      (line  9938)
* \V:                                    I/O.               (line 11719)
* \w:                                    Page Motions.      (line 10020)
* \x:                                    Manipulating Spacing.
                                                            (line  6407)
* \X:                                    Postprocessor Access.
                                                            (line 11739)
* \Y:                                    Postprocessor Access.
                                                            (line 11769)
* \z:                                    Page Motions.      (line 10102)
* \Z:                                    Page Motions.      (line 10107)
* \[:                                    Using Symbols.     (line  7733)
* \\:                                    Copy Mode.         (line  9714)
* \^:                                    Page Motions.      (line 10009)
* \_:                                    Using Symbols.     (line  7841)
* \`:                                    Using Symbols.     (line  7835)
* \{:                                    Conditional Blocks.
                                                            (line  9249)
* \{ <1>:                                Conditional Blocks.
                                                            (line  9250)
* \|:                                    Page Motions.      (line 10004)
* \}:                                    Conditional Blocks.
                                                            (line  9250)
* \~:                                    Manipulating Filling and Adjustment.
                                                            (line  5671)

Appendix D Operator Index
*************************

* Menu:

* !:                                     Numeric Expressions.
                                                             (line 4426)
* %:                                     Numeric Expressions.
                                                             (line 4359)
* &:                                     Numeric Expressions.
                                                             (line 4422)
* (:                                     Numeric Expressions.
                                                             (line 4446)
* ):                                     Numeric Expressions.
                                                             (line 4446)
* *:                                     Numeric Expressions.
                                                             (line 4359)
* +:                                     Numeric Expressions.
                                                             (line 4359)
* + <1>:                                 Numeric Expressions.
                                                             (line 4365)
* + (unary):                             Numeric Expressions.
                                                             (line 4457)
* -:                                     Numeric Expressions.
                                                             (line 4359)
* - <1>:                                 Numeric Expressions.
                                                             (line 4365)
* - (unary):                             Numeric Expressions.
                                                             (line 4457)
* /:                                     Numeric Expressions.
                                                             (line 4359)
* ;:                                     Numeric Expressions.
                                                             (line 4390)
* <:                                     Numeric Expressions.
                                                             (line 4416)
* <=:                                    Numeric Expressions.
                                                             (line 4416)
* <?:                                    Numeric Expressions.
                                                             (line 4407)
* <colon>:                               Numeric Expressions.
                                                             (line 4422)
* =:                                     Numeric Expressions.
                                                             (line 4416)
* ==:                                    Numeric Expressions.
                                                             (line 4416)
* >:                                     Numeric Expressions.
                                                             (line 4416)
* >=:                                    Numeric Expressions.
                                                             (line 4416)
* >?:                                    Numeric Expressions.
                                                             (line 4407)
* |:                                     Numeric Expressions.
                                                             (line 4471)

Appendix E Register Index
*************************

The macro package or program a specific register belongs to is appended
in brackets.

   A register name 'x' consisting of exactly one character can be
accessed as '\nx'.  A register name 'xx' consisting of exactly two
characters can be accessed as '\n(xx'.  Register names 'xxx' of any
length can be accessed as '\n[xxx]'.

* Menu:

* $$:                                    Built-in Registers.
                                                            (line  5571)
* %:                                     Page Layout.       (line  7149)
* % <1>:                                 Page Control.      (line  7178)
* .$:                                    Parameters.        (line  9594)
* .A:                                    Built-in Registers.
                                                            (line  5527)
* .a:                                    Manipulating Spacing.
                                                            (line  6408)
* .b:                                    Artificial Fonts.  (line  8213)
* .br:                                   Control Characters.
                                                            (line  4759)
* .c:                                    Built-in Registers.
                                                            (line  5531)
* .C:                                    Compatibility Mode.
                                                            (line 12415)
* .cdp:                                  Environments.      (line 11387)
* .ce:                                   Manipulating Filling and Adjustment.
                                                            (line  5827)
* .cht:                                  Environments.      (line 11386)
* .color:                                Colors.            (line  8688)
* .cp:                                   Compatibility Mode.
                                                            (line 12426)
* .csk:                                  Environments.      (line 11388)
* .d:                                    Diversions.        (line 11000)
* .ev:                                   Environments.      (line 11309)
* .F:                                    Built-in Registers.
                                                            (line  5536)
* .f:                                    Font Positions.    (line  7551)
* .fam:                                  Font Families.     (line  7464)
* .fn:                                   Selecting Fonts.   (line  7352)
* .fp:                                   Font Positions.    (line  7552)
* .g:                                    Built-in Registers.
                                                            (line  5539)
* .H:                                    Motion Quanta.     (line  4300)
* .h:                                    Diversions.        (line 11007)
* .height:                               Artificial Fonts.  (line  8132)
* .hla:                                  Manipulating Hyphenation.
                                                            (line  6263)
* .hlc:                                  Manipulating Hyphenation.
                                                            (line  6277)
* .hlm:                                  Manipulating Hyphenation.
                                                            (line  6276)
* .hy:                                   Manipulating Hyphenation.
                                                            (line  6056)
* .hym:                                  Manipulating Hyphenation.
                                                            (line  6290)
* .hys:                                  Manipulating Hyphenation.
                                                            (line  6305)
* .i:                                    Line Layout.       (line  6938)
* .in:                                   Line Layout.       (line  6962)
* .int:                                  Line Continuation. (line  7048)
* .j:                                    Manipulating Filling and Adjustment.
                                                            (line  5702)
* .k:                                    Page Motions.      (line 10093)
* .kern:                                 Ligatures and Kerning.
                                                            (line  8289)
* .L:                                    Manipulating Spacing.
                                                            (line  6396)
* .l:                                    Line Layout.       (line  6990)
* .lg:                                   Ligatures and Kerning.
                                                            (line  8271)
* .linetabs:                             Tabs and Fields.   (line  6615)
* .ll:                                   Line Layout.       (line  6991)
* .lt:                                   Page Layout.       (line  7137)
* .m:                                    Colors.            (line  8733)
* .M:                                    Colors.            (line  8761)
* .n:                                    Environments.      (line 11403)
* .ne:                                   Page Location Traps.
                                                            (line 10579)
* .nm:                                   Miscellaneous.     (line 11798)
* .nn:                                   Miscellaneous.     (line 11862)
* .ns:                                   Manipulating Spacing.
                                                            (line  6456)
* .o:                                    Line Layout.       (line  6912)
* .O:                                    Suppressing Output.
                                                            (line 11463)
* .P:                                    Built-in Registers.
                                                            (line  5543)
* .p:                                    Page Layout.       (line  7093)
* .pe:                                   Page Location Traps.
                                                            (line 10598)
* .pn:                                   Page Layout.       (line  7107)
* .ps:                                   Using Fractional Type Sizes.
                                                            (line  8622)
* .psr:                                  Using Fractional Type Sizes.
                                                            (line  8637)
* .pvs:                                  Changing the Vertical Spacing.
                                                            (line  8584)
* .R:                                    Built-in Registers.
                                                            (line  5547)
* .rj:                                   Manipulating Filling and Adjustment.
                                                            (line  5866)
* .s:                                    Changing the Type Size.
                                                            (line  8466)
* .slant:                                Artificial Fonts.  (line  8161)
* .sr:                                   Using Fractional Type Sizes.
                                                            (line  8638)
* .ss:                                   Manipulating Filling and Adjustment.
                                                            (line  5886)
* .sss:                                  Manipulating Filling and Adjustment.
                                                            (line  5887)
* .sty:                                  Font Families.     (line  7505)
* .T:                                    Built-in Registers.
                                                            (line  5551)
* .t:                                    Page Location Traps.
                                                            (line 10520)
* .tabs:                                 Tabs and Fields.   (line  6489)
* .trunc:                                Page Location Traps.
                                                            (line 10587)
* .U:                                    Built-in Registers.
                                                            (line  5555)
* .u:                                    Manipulating Filling and Adjustment.
                                                            (line  5685)
* .V:                                    Motion Quanta.     (line  4301)
* .v:                                    Changing the Vertical Spacing.
                                                            (line  8543)
* .vpt:                                  Vertical Position Traps.
                                                            (line 10407)
* .w:                                    Environments.      (line 11385)
* .warn:                                 Debugging.         (line 12227)
* .x:                                    Built-in Registers.
                                                            (line  5559)
* .y:                                    Built-in Registers.
                                                            (line  5563)
* .Y:                                    Built-in Registers.
                                                            (line  5567)
* .z:                                    Diversions.        (line 10999)
* .zoom:                                 Selecting Fonts.   (line  7421)
* c.:                                    Built-in Registers.
                                                            (line  5532)
* ct:                                    Page Motions.      (line 10025)
* DD [ms]:                               ms Document Control Settings.
                                                            (line  2089)
* DI [ms]:                               ms Document Control Settings.
                                                            (line  2098)
* dl:                                    Diversions.        (line 11026)
* dn:                                    Diversions.        (line 11025)
* dw:                                    Built-in Registers.
                                                            (line  5589)
* dy:                                    Built-in Registers.
                                                            (line  5592)
* FF [ms]:                               ms Document Control Settings.
                                                            (line  2029)
* FI [ms]:                               ms Document Control Settings.
                                                            (line  2022)
* FM [ms]:                               ms Document Control Settings.
                                                            (line  1848)
* FPD [ms]:                              ms Document Control Settings.
                                                            (line  2070)
* FPS [ms]:                              ms Document Control Settings.
                                                            (line  2056)
* FVS [ms]:                              ms Document Control Settings.
                                                            (line  2063)
* GROWPS [ms]:                           ms Document Control Settings.
                                                            (line  1987)
* GS [ms]:                               Differences from AT&T ms.
                                                            (line  3360)
* HM [ms]:                               ms Document Control Settings.
                                                            (line  1841)
* HORPHANS [ms]:                         ms Document Control Settings.
                                                            (line  1998)
* hours:                                 Built-in Registers.
                                                            (line  5586)
* hp:                                    Page Motions.      (line 10090)
* HY [ms]:                               ms Document Control Settings.
                                                            (line  1917)
* LL [ms]:                               ms Document Control Settings.
                                                            (line  1822)
* llx:                                   Miscellaneous.     (line 11946)
* lly:                                   Miscellaneous.     (line 11947)
* ln:                                    Miscellaneous.     (line 11797)
* lsn:                                   Leading Space Traps.
                                                            (line 10790)
* lss:                                   Leading Space Traps.
                                                            (line 10791)
* LT [ms]:                               ms Document Control Settings.
                                                            (line  1831)
* MINGW [ms]:                            ms Document Control Settings.
                                                            (line  2111)
* minutes:                               Built-in Registers.
                                                            (line  5583)
* mo:                                    Built-in Registers.
                                                            (line  5595)
* nl:                                    Page Control.      (line  7240)
* opmaxx:                                Suppressing Output.
                                                            (line 11426)
* opmaxy:                                Suppressing Output.
                                                            (line 11426)
* opminx:                                Suppressing Output.
                                                            (line 11426)
* opminy:                                Suppressing Output.
                                                            (line 11426)
* PD [ms]:                               ms Document Control Settings.
                                                            (line  1946)
* PI [ms]:                               ms Document Control Settings.
                                                            (line  1938)
* PO [ms]:                               ms Document Control Settings.
                                                            (line  1813)
* PORPHANS [ms]:                         ms Document Control Settings.
                                                            (line  1961)
* PS [ms]:                               ms Document Control Settings.
                                                            (line  1903)
* PSINCR [ms]:                           ms Document Control Settings.
                                                            (line  1976)
* QI [ms]:                               ms Document Control Settings.
                                                            (line  1953)
* rsb:                                   Page Motions.      (line 10024)
* rst:                                   Page Motions.      (line 10023)
* sb:                                    Page Motions.      (line 10022)
* seconds:                               Built-in Registers.
                                                            (line  5580)
* skw:                                   Page Motions.      (line 10027)
* slimit:                                Debugging.         (line 12197)
* ssc:                                   Page Motions.      (line 10026)
* st:                                    Page Motions.      (line 10021)
* systat:                                I/O.               (line 11635)
* TC-MARGIN [ms]:                        ms Document Control Settings.
                                                            (line  2119)
* urx:                                   Miscellaneous.     (line 11948)
* ury:                                   Miscellaneous.     (line 11949)
* VS [ms]:                               ms Document Control Settings.
                                                            (line  1910)
* year:                                  Built-in Registers.
                                                            (line  5598)
* yr:                                    Built-in Registers.
                                                            (line  5601)

Appendix F Macro Index
**********************

The macro package a specific macro belongs to is appended in brackets.
They appear without the leading control character (normally '.').

* Menu:

* 1C [ms]:                               ms Multiple Columns.
                                                             (line 3125)
* 2C [ms]:                               ms Multiple Columns.
                                                             (line 3128)
* [ [ms]:                                ms Insertions.      (line 2876)
* ] [ms]:                                ms Insertions.      (line 2877)
* AB [ms]:                               ms Document Description Macros.
                                                             (line 2187)
* AE [ms]:                               ms Document Description Macros.
                                                             (line 2194)
* AI [ms]:                               ms Document Description Macros.
                                                             (line 2170)
* AM [ms]:                               ms Legacy Features. (line 3443)
* AU [ms]:                               ms Document Description Macros.
                                                             (line 2164)
* B [ms]:                                Typeface and decoration.
                                                             (line 2485)
* B1 [ms]:                               ms keeps and displays.
                                                             (line 2774)
* B2 [ms]:                               ms keeps and displays.
                                                             (line 2775)
* BD [ms]:                               ms keeps and displays.
                                                             (line 2815)
* BI [ms]:                               Typeface and decoration.
                                                             (line 2498)
* BT [man]:                              Optional man extensions.
                                                             (line 1442)
* BX [ms]:                               Typeface and decoration.
                                                             (line 2506)
* CD [ms]:                               ms keeps and displays.
                                                             (line 2821)
* CT [man]:                              Optional man extensions.
                                                             (line 1457)
* CW [man]:                              Optional man extensions.
                                                             (line 1460)
* CW [ms]:                               Typeface and decoration.
                                                             (line 2502)
* DA [ms]:                               ms Document Description Macros.
                                                             (line 2177)
* De [man]:                              Optional man extensions.
                                                             (line 1467)
* DE [ms]:                               ms keeps and displays.
                                                             (line 2830)
* Ds [man]:                              Optional man extensions.
                                                             (line 1464)
* DS [ms]:                               ms keeps and displays.
                                                             (line 2805)
* DS [ms] <1>:                           ms keeps and displays.
                                                             (line 2809)
* DS [ms] <2>:                           ms keeps and displays.
                                                             (line 2814)
* DS [ms] <3>:                           ms keeps and displays.
                                                             (line 2820)
* DS [ms] <4>:                           ms keeps and displays.
                                                             (line 2825)
* EE [man]:                              Optional man extensions.
                                                             (line 1474)
* EF [ms]:                               ms Headers and Footers.
                                                             (line 3069)
* EH [ms]:                               ms Headers and Footers.
                                                             (line 3067)
* EN [ms]:                               ms Insertions.      (line 2869)
* EQ [ms]:                               ms Insertions.      (line 2868)
* EX [man]:                              Optional man extensions.
                                                             (line 1470)
* FE [ms]:                               ms Footnotes.       (line 2935)
* FS [ms]:                               ms Footnotes.       (line 2934)
* G [man]:                               Optional man extensions.
                                                             (line 1477)
* GL [man]:                              Optional man extensions.
                                                             (line 1482)
* HB [man]:                              Optional man extensions.
                                                             (line 1487)
* I [ms]:                                Typeface and decoration.
                                                             (line 2495)
* ID [ms]:                               ms keeps and displays.
                                                             (line 2810)
* IP [ms]:                               Paragraphs in ms.   (line 2294)
* KE [ms]:                               ms keeps and displays.
                                                             (line 2762)
* KF [ms]:                               ms keeps and displays.
                                                             (line 2761)
* KS [ms]:                               ms keeps and displays.
                                                             (line 2760)
* LD [ms]:                               ms keeps and displays.
                                                             (line 2806)
* LG [ms]:                               Typeface and decoration.
                                                             (line 2516)
* LP [ms]:                               Paragraphs in ms.   (line 2287)
* MC [ms]:                               ms Multiple Columns.
                                                             (line 3131)
* MS [man]:                              Optional man extensions.
                                                             (line 1495)
* ND [ms]:                               ms Document Description Macros.
                                                             (line 2182)
* NE [man]:                              Optional man extensions.
                                                             (line 1507)
* NH [ms]:                               Headings in ms.     (line 2361)
* NL [ms]:                               Typeface and decoration.
                                                             (line 2528)
* NT [man]:                              Optional man extensions.
                                                             (line 1500)
* OF [ms]:                               ms Headers and Footers.
                                                             (line 3068)
* OH [ms]:                               ms Headers and Footers.
                                                             (line 3066)
* P1 [ms]:                               ms Headers and Footers.
                                                             (line 3078)
* PE [ms]:                               ms Insertions.      (line 2861)
* PF [ms]:                               ms Insertions.      (line 2862)
* PN [man]:                              Optional man extensions.
                                                             (line 1510)
* Pn [man]:                              Optional man extensions.
                                                             (line 1514)
* PP [ms]:                               Paragraphs in ms.   (line 2290)
* PS [ms]:                               ms Insertions.      (line 2860)
* PT [man]:                              Optional man extensions.
                                                             (line 1437)
* PX [ms]:                               ms TOC.             (line 3181)
* QE [ms]:                               Paragraphs in ms.   (line 2307)
* QP [ms]:                               Paragraphs in ms.   (line 2302)
* QS [ms]:                               Paragraphs in ms.   (line 2306)
* R [man]:                               Optional man extensions.
                                                             (line 1520)
* R [ms]:                                Typeface and decoration.
                                                             (line 2491)
* RD [ms]:                               ms keeps and displays.
                                                             (line 2826)
* RE [ms]:                               Indented regions in ms.
                                                             (line 2710)
* RN [man]:                              Optional man extensions.
                                                             (line 1523)
* RP [ms]:                               ms Document Description Macros.
                                                             (line 2145)
* RS [ms]:                               Indented regions in ms.
                                                             (line 2706)
* SH [ms]:                               Headings in ms.     (line 2431)
* SM [ms]:                               Typeface and decoration.
                                                             (line 2522)
* TA [ms]:                               Tab Stops in ms.    (line 3103)
* TB [man]:                              Optional man extensions.
                                                             (line 1492)
* TC [ms]:                               ms TOC.             (line 3186)
* TE [ms]:                               ms Insertions.      (line 2852)
* TL [ms]:                               ms Document Description Macros.
                                                             (line 2159)
* TS [ms]:                               ms Insertions.      (line 2851)
* UL [ms]:                               Typeface and decoration.
                                                             (line 2512)
* VE [man]:                              Optional man extensions.
                                                             (line 1530)
* VS [man]:                              Optional man extensions.
                                                             (line 1526)
* XA [ms]:                               ms TOC.             (line 3170)
* XE [ms]:                               ms TOC.             (line 3171)
* XH [ms]:                               ms TOC.             (line 3222)
* XH-REPLACEMENT [ms]:                   ms TOC.             (line 3231)
* XH-UPDATE-TOC [ms]:                    ms TOC.             (line 3236)
* XN [ms]:                               ms TOC.             (line 3221)
* XN-INIT [ms]:                          ms TOC.             (line 3235)
* XN-REPLACEMENT [ms]:                   ms TOC.             (line 3230)
* XP [ms]:                               Paragraphs in ms.   (line 2313)
* XS [ms]:                               ms TOC.             (line 3169)

Appendix G String Index
***********************

The macro package or program a that defines or uses each string is
appended in brackets.  (Only one string, '.T', is defined by the 'troff'
formatter itself.)  *Note Strings::.

* Menu:

* ! [ms]:                                ms Legacy Features. (line 3482)
* ' [ms]:                                ms Legacy Features. (line 3415)
* ' [ms] <1>:                            ms Legacy Features. (line 3446)
* * [ms]:                                ms Footnotes.       (line 2925)
* , [ms]:                                ms Legacy Features. (line 3433)
* , [ms] <1>:                            ms Legacy Features. (line 3461)
* - [ms]:                                Typographical symbols in ms.
                                                             (line 2266)
* . [ms]:                                ms Legacy Features. (line 3473)
* .T:                                    Strings.            (line 8787)
* .T <1>:                                Strings.            (line 8787)
* / [ms]:                                ms Legacy Features. (line 3464)
* 3 [ms]:                                ms Legacy Features. (line 3491)
* 8 [ms]:                                ms Legacy Features. (line 3485)
* : [ms]:                                ms Legacy Features. (line 3421)
* : [ms] <1>:                            ms Legacy Features. (line 3452)
* < [ms]:                                Typeface and decoration.
                                                             (line 2562)
* > [ms]:                                Typeface and decoration.
                                                             (line 2563)
* ? [ms]:                                ms Legacy Features. (line 3479)
* ^ [ms]:                                ms Legacy Features. (line 3424)
* ^ [ms] <1>:                            ms Legacy Features. (line 3455)
* _ [ms]:                                ms Legacy Features. (line 3470)
* ` [ms]:                                ms Legacy Features. (line 3418)
* ` [ms] <1>:                            ms Legacy Features. (line 3449)
* { [ms]:                                Typeface and decoration.
                                                             (line 2558)
* } [ms]:                                Typeface and decoration.
                                                             (line 2559)
* ~ [ms]:                                ms Legacy Features. (line 3427)
* ~ [ms] <1>:                            ms Legacy Features. (line 3458)
* ABSTRACT [ms]:                         ms language and localization.
                                                             (line 3019)
* ae [ms]:                               ms Legacy Features. (line 3506)
* Ae [ms]:                               ms Legacy Features. (line 3509)
* C [ms]:                                ms Legacy Features. (line 3430)
* CF [ms]:                               ms Document Control Settings.
                                                             (line 1886)
* CH [ms]:                               ms Document Control Settings.
                                                             (line 1865)
* d- [ms]:                               ms Legacy Features. (line 3494)
* D- [ms]:                               ms Legacy Features. (line 3497)
* FAM [ms]:                              ms Document Control Settings.
                                                             (line 1926)
* FR [ms]:                               ms Document Control Settings.
                                                             (line 2077)
* LF [ms]:                               ms Document Control Settings.
                                                             (line 1879)
* LH [ms]:                               ms Document Control Settings.
                                                             (line 1858)
* MONTH1 [ms]:                           ms language and localization.
                                                             (line 3028)
* MONTH10 [ms]:                          ms language and localization.
                                                             (line 3037)
* MONTH11 [ms]:                          ms language and localization.
                                                             (line 3038)
* MONTH12 [ms]:                          ms language and localization.
                                                             (line 3039)
* MONTH2 [ms]:                           ms language and localization.
                                                             (line 3029)
* MONTH3 [ms]:                           ms language and localization.
                                                             (line 3030)
* MONTH4 [ms]:                           ms language and localization.
                                                             (line 3031)
* MONTH5 [ms]:                           ms language and localization.
                                                             (line 3032)
* MONTH6 [ms]:                           ms language and localization.
                                                             (line 3033)
* MONTH7 [ms]:                           ms language and localization.
                                                             (line 3034)
* MONTH8 [ms]:                           ms language and localization.
                                                             (line 3035)
* MONTH9 [ms]:                           ms language and localization.
                                                             (line 3036)
* o [ms]:                                ms Legacy Features. (line 3476)
* oe [ms]:                               ms Legacy Features. (line 3512)
* OE [ms]:                               ms Legacy Features. (line 3515)
* Q [ms]:                                Typographical symbols in ms.
                                                             (line 2269)
* q [ms]:                                ms Legacy Features. (line 3488)
* REFERENCES [ms]:                       ms language and localization.
                                                             (line 3014)
* RF [ms]:                               ms Document Control Settings.
                                                             (line 1893)
* RH [ms]:                               ms Document Control Settings.
                                                             (line 1872)
* SN [ms]:                               Headings in ms.     (line 2410)
* SN-DOT [ms]:                           Headings in ms.     (line 2408)
* SN-NO-DOT [ms]:                        Headings in ms.     (line 2409)
* SN-STYLE [ms]:                         ms Document Control Settings.
                                                             (line 2011)
* SN-STYLE [ms] <1>:                     Headings in ms.     (line 2407)
* th [ms]:                               ms Legacy Features. (line 3500)
* Th [ms]:                               ms Legacy Features. (line 3503)
* TOC [ms]:                              ms language and localization.
                                                             (line 3024)
* U [ms]:                                Typographical symbols in ms.
                                                             (line 2270)
* v [ms]:                                ms Legacy Features. (line 3467)

Appendix H File Keyword Index
*****************************

* Menu:

* #:                                     DESC File Format.  (line 13432)
* # <1>:                                 Font Description File Format.
                                                            (line 13592)
* ---:                                   Font Description File Format.
                                                            (line 13639)
* biggestfont:                           DESC File Format.  (line 13562)
* charset:                               DESC File Format.  (line 13557)
* charset <1>:                           Font Description File Format.
                                                            (line 13631)
* family:                                Selecting Fonts.   (line  7352)
* family <1>:                            DESC File Format.  (line 13436)
* fonts:                                 Using Symbols.     (line  7612)
* fonts <1>:                             Special Fonts.     (line  8104)
* fonts <2>:                             DESC File Format.  (line 13439)
* hor:                                   DESC File Format.  (line 13445)
* image_generator:                       DESC File Format.  (line 13449)
* kernpairs:                             Font Description File Format.
                                                            (line 13727)
* ligatures:                             Font Description File Format.
                                                            (line 13611)
* name:                                  Font Description File Format.
                                                            (line 13596)
* paperlength:                           DESC File Format.  (line 13455)
* papersize:                             DESC File Format.  (line 13459)
* paperwidth:                            DESC File Format.  (line 13486)
* pass_filenames:                        DESC File Format.  (line 13490)
* postpro:                               DESC File Format.  (line 13495)
* prepro:                                DESC File Format.  (line 13498)
* print:                                 DESC File Format.  (line 13502)
* res:                                   DESC File Format.  (line 13506)
* sizes:                                 DESC File Format.  (line 13509)
* sizescale:                             DESC File Format.  (line 13514)
* slant:                                 Font Description File Format.
                                                            (line 13607)
* spacewidth:                            Font Description File Format.
                                                            (line 13601)
* spare1:                                DESC File Format.  (line 13562)
* spare2:                                DESC File Format.  (line 13562)
* special:                               Artificial Fonts.  (line  8230)
* special <1>:                           Font Description File Format.
                                                            (line 13618)
* styles:                                Selecting Fonts.   (line  7352)
* styles <1>:                            Font Families.     (line  7511)
* styles <2>:                            DESC File Format.  (line 13518)
* tcommand:                              DESC File Format.  (line 13522)
* unicode:                               DESC File Format.  (line 13526)
* unitwidth:                             DESC File Format.  (line 13540)
* unscaled_charwidths:                   DESC File Format.  (line 13544)
* use_charnames_in_special:              Postprocessor Access.
                                                            (line 11760)
* use_charnames_in_special <1>:          DESC File Format.  (line 13548)
* vert:                                  DESC File Format.  (line 13553)

Appendix I Program and File Index
*********************************

* Menu:

* an.tmac:                               man.               (line  1417)
* changebar:                             Miscellaneous.     (line 11940)
* composite.tmac:                        Using Symbols.     (line  7802)
* cp1047.tmac:                           Input Encodings.   (line  3986)
* cs.tmac:                               Manipulating Hyphenation.
                                                            (line  6215)
* de.tmac:                               Manipulating Hyphenation.
                                                            (line  6215)
* DESC:                                  Selecting Fonts.   (line  7352)
* DESC <1>:                              Font Families.     (line  7511)
* DESC <2>:                              Using Symbols.     (line  7612)
* DESC <3>:                              Using Symbols.     (line  7820)
* DESC <4>:                              Special Fonts.     (line  8104)
* diffmk:                                Miscellaneous.     (line 11940)
* ec.tmac:                               Input Encodings.   (line  4027)
* en.tmac:                               Manipulating Hyphenation.
                                                            (line  6215)
* eqn:                                   ms Insertions.     (line  2845)
* fr.tmac:                               Manipulating Hyphenation.
                                                            (line  6215)
* freeeuro.pfa:                          Input Encodings.   (line  4027)
* gchem:                                 Groff Options.     (line   478)
* gdiffmk:                               Miscellaneous.     (line 11940)
* geqn:                                  Groff Options.     (line   478)
* ggrn:                                  Groff Options.     (line   478)
* gpic:                                  Groff Options.     (line   478)
* grap:                                  Groff Options.     (line   478)
* grefer:                                Groff Options.     (line   478)
* groff:                                 Groff Options.     (line   478)
* gsoelim:                               Groff Options.     (line   478)
* gtbl:                                  Groff Options.     (line   478)
* gtroff:                                Groff Options.     (line   478)
* it.tmac:                               Manipulating Hyphenation.
                                                            (line  6215)
* ja.tmac:                               Manipulating Hyphenation.
                                                            (line  6215)
* latin1.tmac:                           Input Encodings.   (line  3991)
* latin2.tmac:                           Input Encodings.   (line  4004)
* latin5.tmac:                           Input Encodings.   (line  4010)
* latin9.tmac:                           Input Encodings.   (line  4015)
* makeindex:                             Indexing.          (line  1340)
* man.local:                             Optional man extensions.
                                                            (line  1427)
* man.tmac:                              man.               (line  1417)
* man.ultrix:                            Optional man extensions.
                                                            (line  1451)
* nrchbar:                               Miscellaneous.     (line 11940)
* papersize.tmac:                        Paper Format.      (line   975)
* perl:                                  I/O.               (line 11645)
* pic:                                   ms Insertions.     (line  2845)
* post-grohtml:                          Groff Options.     (line   766)
* pre-grohtml:                           Groff Options.     (line   766)
* preconv:                               Groff Options.     (line   478)
* refer:                                 ms Insertions.     (line  2845)
* soelim:                                Debugging.         (line 12103)
* sv.tmac:                               Manipulating Hyphenation.
                                                            (line  6215)
* tbl:                                   ms Insertions.     (line  2845)
* trace.tmac:                            Writing Macros.    (line  9551)
* troffrc:                               Groff Options.     (line   699)
* troffrc <1>:                           Paper Format.      (line   975)
* troffrc <2>:                           Manipulating Hyphenation.
                                                            (line  6215)
* troffrc <3>:                           Manipulating Hyphenation.
                                                            (line  6263)
* troffrc <4>:                           troff and nroff Modes.
                                                            (line  6832)
* troffrc-end:                           Groff Options.     (line   699)
* troffrc-end <1>:                       Manipulating Hyphenation.
                                                            (line  6263)
* troffrc-end <2>:                       troff and nroff Modes.
                                                            (line  6832)
* tty.tmac:                              troff and nroff Modes.
                                                            (line  6840)
* tty.tmac <1>:                          Line Layout.       (line  6912)
* vtroff:                                Operators in Conditionals.
                                                            (line  9070)
* zh.tmac:                               Manipulating Hyphenation.
                                                            (line  6215)

Appendix J Concept Index
************************

* Menu:

* ", as delimiter:                       Delimiters.        (line  5060)
* ", at end of sentence:                 Sentences.         (line  3663)
* ", at end of sentence <1>:             Using Symbols.     (line  7892)
* ", embedding in a macro argument:      Calling Macros.    (line  4875)
* %, as delimiter:                       Delimiters.        (line  5092)
* &, as delimiter:                       Delimiters.        (line  5092)
* ', as a comment:                       Comments.          (line  5164)
* ', as delimiter:                       Delimiters.        (line  5060)
* ', at end of sentence:                 Sentences.         (line  3663)
* ', at end of sentence <1>:             Using Symbols.     (line  7892)
* (, as delimiter:                       Delimiters.        (line  5092)
* ), as delimiter:                       Delimiters.        (line  5092)
* ), at end of sentence:                 Sentences.         (line  3663)
* ), at end of sentence <1>:             Using Symbols.     (line  7892)
* *, as delimiter:                       Delimiters.        (line  5092)
* *, at end of sentence:                 Sentences.         (line  3663)
* *, at end of sentence <1>:             Using Symbols.     (line  7892)
* +, and page motion:                    Numeric Expressions.
                                                            (line  4457)
* +, as delimiter:                       Delimiters.        (line  5092)
* -, and page motion:                    Numeric Expressions.
                                                            (line  4457)
* -, as delimiter:                       Delimiters.        (line  5092)
* ., as delimiter:                       Delimiters.        (line  5090)
* .h register, difference from nl:       Diversions.        (line 11020)
* .ps register, in comparison with .psr: Using Fractional Type Sizes.
                                                            (line  8638)
* .s register, in comparison with .sr:   Using Fractional Type Sizes.
                                                            (line  8638)
* .S register, Plan 9 alias for .tabs:   Tabs and Fields.   (line  6593)
* .t register, and diversions:           Diversion Traps.   (line 10668)
* .tabs register, Plan 9 alias (.S):     Tabs and Fields.   (line  6593)
* .V register, and vs:                   Changing the Vertical Spacing.
                                                            (line  8546)
* /, as delimiter:                       Delimiters.        (line  5092)
* 8-bit input:                           Font Description File Format.
                                                            (line 13639)
* <, as delimiter:                       Delimiters.        (line  5092)
* <colon>, as delimiter:                 Delimiters.        (line  5092)
* =, as delimiter:                       Delimiters.        (line  5092)
* >, as delimiter:                       Delimiters.        (line  5092)
* [, macro names starting with, and refer: Identifiers.     (line  4623)
* \!, and copy mode:                     Diversions.        (line 11074)
* \!, and output request:                Diversions.        (line 11106)
* \!, and trnt:                          Character Translations.
                                                            (line  6795)
* \!, as delimiter:                      Delimiters.        (line  5069)
* \!, as delimiter <1>:                  Delimiters.        (line  5096)
* \!, in top-level diversion:            Diversions.        (line 11098)
* \!, incompatibilities with AT&T troff: Other Differences. (line 12546)
* \!, incompatibilities with AT&T troff <1>: Other Differences.
                                                            (line 12636)
* \$, when reading text for a macro:     Copy Mode.         (line  9699)
* \%, and translations:                  Character Translations.
                                                            (line  6747)
* \%, as delimiter:                      Delimiters.        (line  5069)
* \%, as delimiter <1>:                  Delimiters.        (line  5096)
* \%, following \X or \Y:                Manipulating Hyphenation.
                                                            (line  6001)
* \%, in \X:                             Postprocessor Access.
                                                            (line 11746)
* \%, incompatibilities with AT&T troff: Other Differences. (line 12546)
* \&, and glyph definitions:             Using Symbols.     (line  7948)
* \&, and translations:                  Character Translations.
                                                            (line  6758)
* \&, as delimiter:                      Delimiters.        (line  5069)
* \&, at end of sentence:                Sentences.         (line  3647)
* \&, in \X:                             Postprocessor Access.
                                                            (line 11746)
* \&, incompatibilities with AT&T troff: Other Differences. (line 12546)
* \', and translations:                  Character Translations.
                                                            (line  6741)
* \', as delimiter:                      Delimiters.        (line  5069)
* \', as delimiter <1>:                  Delimiters.        (line  5096)
* \', incompatibilities with AT&T troff: Other Differences. (line 12546)
* \(, and translations:                  Character Translations.
                                                            (line  6741)
* \), as delimiter:                      Delimiters.        (line  5069)
* \), in \X:                             Postprocessor Access.
                                                            (line 11746)
* \*, and warnings:                      Warnings.          (line 12301)
* \*, incompatibilities with AT&T troff: Compatibility Mode.
                                                            (line 12404)
* \*, when reading text for a macro:     Copy Mode.         (line  9699)
* \, disabling (eo):                     Using Escape Sequences.
                                                            (line  5007)
* \, embedding in a macro argument:      Calling Macros.    (line  4875)
* \,, as delimiter:                      Delimiters.        (line  5069)
* \- glyph, and cflags:                  Using Symbols.     (line  7875)
* \-, and translations:                  Character Translations.
                                                            (line  6741)
* \-, as delimiter:                      Delimiters.        (line  5069)
* \-, as delimiter <1>:                  Delimiters.        (line  5096)
* \-, incompatibilities with AT&T troff: Other Differences. (line 12546)
* \/, as delimiter:                      Delimiters.        (line  5069)
* \/, as delimiter <1>:                  Delimiters.        (line  5096)
* \0, as delimiter:                      Delimiters.        (line  5069)
* \:, as delimiter:                      Delimiters.        (line  5069)
* \:, as delimiter <1>:                  Delimiters.        (line  5096)
* \<colon>, in \X:                       Postprocessor Access.
                                                            (line 11746)
* \?, and copy mode:                     Operators in Conditionals.
                                                            (line  9116)
* \?, and copy mode <1>:                 Diversions.        (line 11074)
* \?, as delimiter:                      Delimiters.        (line  5069)
* \?, in top-level diversion:            Diversions.        (line 11103)
* \?, incompatibilities with AT&T troff: Other Differences. (line 12636)
* \a, and copy mode:                     Leaders.           (line  6658)
* \a, and translations:                  Character Translations.
                                                            (line  6751)
* \a, as delimiter:                      Delimiters.        (line  5069)
* \A, delimiters allowed by:             Delimiters.        (line  5076)
* \A, incompatibilities with AT&T troff: Other Differences. (line 12546)
* \b, delimiters allowed by:             Delimiters.        (line  5076)
* \b, limitations of:                    Drawing Geometric Objects.
                                                            (line 10316)
* \C, and translations:                  Character Translations.
                                                            (line  6741)
* \c, as delimiter:                      Delimiters.        (line  5069)
* \c, as delimiter <1>:                  Delimiters.        (line  5096)
* \c, incompatibilities with AT&T troff: Other Differences. (line 12546)
* \c, when filling disabled:             Line Continuation. (line  7062)
* \c, when filling enabled:              Line Continuation. (line  7054)
* \d, as delimiter:                      Delimiters.        (line  5069)
* \D, delimiters allowed by:             Delimiters.        (line  5087)
* \e, and glyph definitions:             Using Symbols.     (line  7948)
* \e, and translations:                  Character Translations.
                                                            (line  6745)
* \e, as delimiter:                      Delimiters.        (line  5069)
* \E, as delimiter:                      Delimiters.        (line  5069)
* \e, as delimiter <1>:                  Delimiters.        (line  5096)
* \e, incompatibilities with AT&T troff: Other Differences. (line 12636)
* \F, and changing fonts:                Selecting Fonts.   (line  7352)
* \f, and font translations:             Selecting Fonts.   (line  7406)
* \f, incompatibilities with AT&T troff: Compatibility Mode.
                                                            (line 12506)
* \h, delimiters allowed by:             Delimiters.        (line  5087)
* \H, delimiters allowed by:             Delimiters.        (line  5087)
* \H, incompatibilities with AT&T troff: Compatibility Mode.
                                                            (line 12506)
* \H, using + and - with:                Numeric Expressions.
                                                            (line  4463)
* \H, with fractional type sizes:        Using Fractional Type Sizes.
                                                            (line  8604)
* \l, and glyph definitions:             Using Symbols.     (line  7948)
* \L, and glyph definitions:             Using Symbols.     (line  7948)
* \l, delimiters allowed by:             Delimiters.        (line  5087)
* \L, delimiters allowed by:             Delimiters.        (line  5087)
* \N, and translations:                  Character Translations.
                                                            (line  6741)
* \n, and warnings:                      Warnings.          (line 12313)
* \N, delimiters allowed by:             Delimiters.        (line  5087)
* \n, incompatibilities with AT&T troff: Compatibility Mode.
                                                            (line 12404)
* \n, when reading text for a macro:     Copy Mode.         (line  9699)
* \o, delimiters allowed by:             Delimiters.        (line  5076)
* \p, as delimiter:                      Delimiters.        (line  5069)
* \p, as delimiter <1>:                  Delimiters.        (line  5096)
* \R, after \c:                          Line Continuation. (line  7048)
* \R, and warnings:                      Warnings.          (line 12313)
* \r, as delimiter:                      Delimiters.        (line  5069)
* \R, delimiters allowed by:             Delimiters.        (line  5087)
* \R, difference from nr:                Auto-increment.    (line  5372)
* \R, using + and - with:                Numeric Expressions.
                                                            (line  4463)
* \<RET>, when reading text for a macro: Copy Mode.         (line  9699)
* \s, delimiters allowed by:             Delimiters.        (line  5087)
* \S, delimiters allowed by:             Delimiters.        (line  5087)
* \s, incompatibilities with AT&T troff: Compatibility Mode.
                                                            (line 12506)
* \S, incompatibilities with AT&T troff: Compatibility Mode.
                                                            (line 12506)
* \s, incompatibilities with AT&T troff <1>: Compatibility Mode.
                                                            (line 12519)
* \s, using + and - with:                Numeric Expressions.
                                                            (line  4463)
* \s, with fractional type sizes:        Using Fractional Type Sizes.
                                                            (line  8604)
* \<SP>, as delimiter:                   Delimiters.        (line  5069)
* \<SP>, difference from \~:             Calling Macros.    (line  4867)
* \<SP>, incompatibilities with AT&T troff: Other Differences.
                                                            (line 12546)
* \t, and copy mode:                     Tabs and Fields.   (line  6485)
* \t, and translations:                  Character Translations.
                                                            (line  6751)
* \t, and warnings:                      Warnings.          (line 12319)
* \t, as delimiter:                      Delimiters.        (line  5069)
* \u, as delimiter:                      Delimiters.        (line  5069)
* \V, and copy mode:                     I/O.               (line 11721)
* \v, delimiters allowed by:             Delimiters.        (line  5087)
* \v, internal representation:           Gtroff Internals.  (line 12017)
* \w, delimiters allowed by:             Delimiters.        (line  5076)
* \X, and special characters:            Postprocessor Access.
                                                            (line 11760)
* \X, delimiters allowed by:             Delimiters.        (line  5076)
* \x, delimiters allowed by:             Delimiters.        (line  5087)
* \X, followed by \%:                    Manipulating Hyphenation.
                                                            (line  6001)
* \Y, followed by \%:                    Manipulating Hyphenation.
                                                            (line  6001)
* \Z, delimiters allowed by:             Delimiters.        (line  5076)
* \[, and translations:                  Character Translations.
                                                            (line  6741)
* \\, when reading text for a macro:     Copy Mode.         (line  9738)
* \^, as delimiter:                      Delimiters.        (line  5069)
* \^, incompatibilities with AT&T troff: Other Differences. (line 12546)
* \_, and translations:                  Character Translations.
                                                            (line  6741)
* \_, as delimiter:                      Delimiters.        (line  5069)
* \_, as delimiter <1>:                  Delimiters.        (line  5096)
* \_, incompatibilities with AT&T troff: Other Differences. (line 12546)
* \`, and translations:                  Character Translations.
                                                            (line  6741)
* \`, as delimiter:                      Delimiters.        (line  5069)
* \`, as delimiter <1>:                  Delimiters.        (line  5096)
* \`, incompatibilities with AT&T troff: Other Differences. (line 12546)
* \{, as delimiter:                      Delimiters.        (line  5069)
* \{, as delimiter <1>:                  Delimiters.        (line  5096)
* \{, incompatibilities with AT&T troff: Other Differences. (line 12546)
* \|, as delimiter:                      Delimiters.        (line  5069)
* \|, incompatibilities with AT&T troff: Other Differences. (line 12546)
* \}, and warnings:                      Warnings.          (line 12324)
* \}, as delimiter:                      Delimiters.        (line  5069)
* \}, as delimiter <1>:                  Delimiters.        (line  5096)
* \}, incompatibilities with AT&T troff: Other Differences. (line 12546)
* \~, and translations:                  Character Translations.
                                                            (line  6747)
* \~, as delimiter:                      Delimiters.        (line  5069)
* \~, difference from \<SP>:             Calling Macros.    (line  4867)
* \~, incompatibilities with AT&T troff: Other Differences. (line 12539)
* ], as part of an identifier:           Identifiers.       (line  4616)
* ], at end of sentence:                 Sentences.         (line  3663)
* ], at end of sentence <1>:             Using Symbols.     (line  7892)
* ], macro names starting with, and refer: Identifiers.     (line  4623)
* |, and page motion:                    Numeric Expressions.
                                                            (line  4471)
* ab request, incompatibilities with AT&T troff: Other Differences.
                                                            (line 12592)
* aborting (ab):                         Debugging.         (line 12127)
* absolute (sic) position operator (|):  Numeric Expressions.
                                                            (line  4471)
* abstract font style:                   Using Fonts.       (line  7302)
* abstract font style, setting up (sty): Font Families.     (line  7505)
* accent marks [ms]:                     ms Legacy Features.
                                                            (line  3405)
* access to postprocessor:               Postprocessor Access.
                                                            (line 11729)
* accessing unnamed glyphs with \N:      Font Description File Format.
                                                            (line 13639)
* activating kerning (kern):             Ligatures and Kerning.
                                                            (line  8289)
* activating ligatures (lg):             Ligatures and Kerning.
                                                            (line  8271)
* activating track kerning (tkf):        Ligatures and Kerning.
                                                            (line  8307)
* ad request, and hyphenation margin:    Manipulating Hyphenation.
                                                            (line  6290)
* ad request, and hyphenation space:     Manipulating Hyphenation.
                                                            (line  6305)
* addition:                              Numeric Expressions.
                                                            (line  4359)
* additional inter-sentence space:       Manipulating Filling and Adjustment.
                                                            (line  5893)
* adjustment and filling, manipulating:  Manipulating Filling and Adjustment.
                                                            (line  5624)
* adjustment mode register (.j):         Manipulating Filling and Adjustment.
                                                            (line  5731)
* adjustment to both margins, difference from AT&T troff: Other Differences.
                                                            (line 12552)
* Adobe Glyph List (AGL):                Using Symbols.     (line  7688)
* alias, diversion, creating (als):      Strings.           (line  8972)
* alias, diversion, removing (rm):       Strings.           (line  9007)
* alias, macro, creating (als):          Strings.           (line  8972)
* alias, macro, removing (rm):           Strings.           (line  9007)
* alias, register, creating (aln):       Setting Registers. (line  5323)
* alias, register, removing (rr):        Setting Registers. (line  5329)
* alias, string, creating (als):         Strings.           (line  8972)
* alias, string, removing (rm):          Strings.           (line  9007)
* aliasing fonts with third argument to fp request: Font Positions.
                                                            (line  7561)
* als request, and \$0:                  Parameters.        (line  9657)
* am, am1, ami requests, and warnings:   Warnings.          (line 12301)
* appending to a diversion (da, boxa):   Diversions.        (line 10944)
* appending to a file (opena):           I/O.               (line 11671)
* appending to a macro (am):             Writing Macros.    (line  9533)
* appending to a string (as):            Strings.           (line  8889)
* approximation output register (.A):    Built-in Registers.
                                                            (line  5527)
* arc, drawing (\D'a ...'):              Drawing Geometric Objects.
                                                            (line 10225)
* argument:                              Requests and Macros.
                                                            (line  3829)
* arguments to macros:                   Calling Macros.    (line  4851)
* arguments to macros, and tabs:         Invoking Requests. (line  4789)
* arguments to requests:                 Invoking Requests. (line  4789)
* arguments to requests, and tabs:       Invoking Requests. (line  4789)
* arguments, and compatibility mode:     Gtroff Internals.  (line 12054)
* arguments, to escape sequences, delimiting: Delimiters.   (line  5060)
* arguments, to strings:                 Strings.           (line  8802)
* arithmetic operators:                  Numeric Expressions.
                                                            (line  4359)
* artificial fonts:                      Artificial Fonts.  (line  8122)
* as, as1 requests, and comments:        Comments.          (line  5139)
* as, as1 requests, and warnings:        Warnings.          (line 12301)
* ASCII output encoding:                 Groff Options.     (line   743)
* asciify request, and writem:           I/O.               (line 11694)
* assertion (arithmetic operator):       Numeric Expressions.
                                                            (line  4365)
* assign number format to register (af): Assigning Register Formats.
                                                            (line  5413)
* assignments, indirect:                 Interpolating Registers.
                                                            (line  5340)
* assignments, nested:                   Interpolating Registers.
                                                            (line  5340)
* AT&T ms, macro package differences:    Differences from AT&T ms.
                                                            (line  3276)
* attributes, character cell:            Using Fonts.       (line  7323)
* auto-incrementation of a register:     Auto-increment.    (line  5364)
* automatic font mounting:               Selecting Fonts.   (line  7365)
* automatic hyphenation:                 Manipulating Hyphenation.
                                                            (line  5941)
* automatic hyphenation parameters:      Manipulating Hyphenation.
                                                            (line  6042)
* auxiliary macro package:               Major Macro Packages.
                                                            (line  1408)
* available glyphs, list of (groff_char(7) man page): Using Symbols.
                                                            (line  7675)
* background:                            Background.        (line   229)
* background color name register (.M):   Colors.            (line  8765)
* backslash glyph, formatting (\[rs]):   Using Escape Sequences.
                                                            (line  5002)
* backslash, embedding in a macro argument: Calling Macros. (line  4875)
* backslash, printing (\\, \e, \E, \[rs]): Other Differences.
                                                            (line 12636)
* backspace character, and translations: Character Translations.
                                                            (line  6751)
* backtrace of input stack (backtrace):  Debugging.         (line 12176)
* baseline rule special character(\[ru]): Drawing Geometric Objects.
                                                            (line 10146)
* baseline, text:                        Page Geometry.     (line  4184)
* baseline, text <1>:                    Manipulating Type Size and Vertical Spacing.
                                                            (line  8438)
* basic scaling unit (u):                Measurements.      (line  4245)
* basic units:                           Page Geometry.     (line  4169)
* basic units, conversion to:            Measurements.      (line  4239)
* basics of macro package usage:         Basics.            (line  1058)
* bd request, and font styles:           Font Families.     (line  7505)
* bd request, and font translations:     Selecting Fonts.   (line  7406)
* bd request, incompatibilities with AT&T troff: Other Differences.
                                                            (line 12609)
* beginning diversion (di, box):         Diversions.        (line 10944)
* beginning of conditional block (\{):   Conditional Blocks.
                                                            (line  9250)
* blank line:                            Breaking.          (line  3749)
* blank line macro (blm):                Breaking.          (line  3749)
* blank line macro (blm) <1>:            Invoking Requests. (line  4820)
* blank line macro (blm) <2>:            Blank Line Traps.  (line 10779)
* blank line trap (blm):                 Invoking Requests. (line  4820)
* blank line traps:                      Blank Line Traps.  (line 10778)
* blank lines, disabling:                Manipulating Spacing.
                                                            (line  6456)
* block, conditional, beginning (\{):    Conditional Blocks.
                                                            (line  9250)
* block, conditional, end (\}):          Conditional Blocks.
                                                            (line  9250)
* blocks, conditional:                   Conditional Blocks.
                                                            (line  9243)
* body, of a while request:              while.             (line  9330)
* boldface, imitating (bd):              Artificial Fonts.  (line  8213)
* bottom margin:                         Page Location Traps.
                                                            (line 10440)
* boundary-relative motion operator (|): Numeric Expressions.
                                                            (line  4471)
* bounding box:                          Miscellaneous.     (line 11949)
* box (diversion operation):             Diversions.        (line 10971)
* box request, and warnings:             Warnings.          (line 12296)
* box rule glyph (\[br]):                Drawing Geometric Objects.
                                                            (line 10168)
* box, boxa requests, and warnings:      Warnings.          (line 12301)
* boxa request, and dn (dl):             Diversions.        (line 11026)
* boxa request, and warnings:            Warnings.          (line 12296)
* boxes [ms]:                            ms keeps and displays.
                                                            (line  2771)
* bp request, and top-level diversion:   Page Control.      (line  7183)
* bp request, and traps (.pe):           Page Location Traps.
                                                            (line 10598)
* bp request, causing implicit break:    Manipulating Filling and Adjustment.
                                                            (line  5624)
* bp request, incompatibilities with AT&T troff: Other Differences.
                                                            (line 12596)
* bp request, using + and - with:        Numeric Expressions.
                                                            (line  4463)
* br glyph, and cflags:                  Using Symbols.     (line  7887)
* brace escape sequence, closing (\}):   Conditional Blocks.
                                                            (line  9250)
* brace escape sequence, opening (\}):   Conditional Blocks.
                                                            (line  9250)
* brace escape sequences (\{, \}):       Conditional Blocks.
                                                            (line  9250)
* break:                                 Breaking.          (line  3725)
* break <1>:                             Manipulating Filling and Adjustment.
                                                            (line  5624)
* break (introduction):                  Basics.            (line  1188)
* break request, in a while loop:        while.             (line  9393)
* break, page:                           Page Geometry.     (line  4208)
* break, page <1>:                       Page Control.      (line  7170)
* break, page <2>:                       The Implicit Page Trap.
                                                            (line 10638)
* break, page (introduction):            Basics.            (line  1185)
* break, page, final:                    End-of-input Traps.
                                                            (line 10834)
* break, page, prevented by vpt:         Vertical Position Traps.
                                                            (line 10414)
* breaking file names (\:):              Manipulating Hyphenation.
                                                            (line  6008)
* breaking URLs (\:):                    Manipulating Hyphenation.
                                                            (line  6008)
* breaking without hyphens (\:):         Manipulating Hyphenation.
                                                            (line  6008)
* built-in register, removing:           Built-in Registers.
                                                            (line  5513)
* built-in registers:                    Built-in Registers.
                                                            (line  5508)
* bulleted list, example markup [ms]:    Lists in ms.       (line  2590)
* c scaling unit:                        Measurements.      (line  4252)
* calling a macro:                       Requests and Macros.
                                                            (line  3840)
* calling macros:                        Calling Macros.    (line  4851)
* capabilities of groff:                 groff Capabilities.
                                                            (line   281)
* case-transforming a string (stringdown, stringup): Strings.
                                                            (line  8945)
* categories, warning:                   Warnings.          (line 12250)
* CCSID 1047 output encoding (EBCDIC):   Groff Options.     (line   755)
* ce request, causing implicit break:    Manipulating Filling and Adjustment.
                                                            (line  5624)
* ce request, difference from .ad c:     Manipulating Filling and Adjustment.
                                                            (line  5836)
* cell, character, attributes:           Using Fonts.       (line  7323)
* centered text (filled):                Manipulating Filling and Adjustment.
                                                            (line  5716)
* centered text (unfilled):              Manipulating Filling and Adjustment.
                                                            (line  5827)
* centering lines (ce):                  Manipulating Filling and Adjustment.
                                                            (line  5827)
* centering lines (introduction):        Basics.            (line  1170)
* centimeter scaling unit (c):           Measurements.      (line  4252)
* cf request, and copy mode:             I/O.               (line 11521)
* cf request, causing implicit break:    Manipulating Filling and Adjustment.
                                                            (line  5624)
* changing control characters:           Control Characters.
                                                            (line  4726)
* changing font family (fam, \F):        Font Families.     (line  7467)
* changing fonts (ft, \f):               Selecting Fonts.   (line  7352)
* changing format, and read-only registers: Assigning Register Formats.
                                                            (line  5472)
* changing the font height (\H):         Artificial Fonts.  (line  8132)
* changing the font slant (\S):          Artificial Fonts.  (line  8161)
* changing the page number character (pc): Page Layout.     (line  7149)
* changing trap location (ch):           Page Location Traps.
                                                            (line 10528)
* changing type sizes (ps, \s):          Changing the Type Size.
                                                            (line  8466)
* changing vertical line spacing (vs):   Changing the Vertical Spacing.
                                                            (line  8543)
* char request, and soft hyphen character: Manipulating Hyphenation.
                                                            (line  6032)
* char request, and translations:        Character Translations.
                                                            (line  6741)
* char request, used with \N:            Using Symbols.     (line  7810)
* character:                             Using Symbols.     (line  7600)
* character cell attributes:             Using Fonts.       (line  7323)
* character class (class):               Character Classes. (line  8034)
* character classes:                     Character Classes. (line  8028)
* character properties (cflags):         Using Symbols.     (line  7846)
* character translations:                Character Translations.
                                                            (line  6722)
* character, backspace, and translations: Character Translations.
                                                            (line  6751)
* character, control (.):                Requests and Macros.
                                                            (line  3813)
* character, control, changing (cc):     Control Characters.
                                                            (line  4738)
* character, defining (char):            Using Symbols.     (line  7948)
* character, defining fallback (fchar, fschar, schar): Using Symbols.
                                                            (line  7948)
* character, distinguished from glyph:   Using Symbols.     (line  7600)
* character, dummy (\&):                 Dummy Characters.  (line  8357)
* character, dummy (\&), as control character suppressor: Requests and Macros.
                                                            (line  3820)
* character, dummy (\&), effect on kerning: Ligatures and Kerning.
                                                            (line  8294)
* character, dummy (\&), effect on \l escape sequence: Drawing Geometric Objects.
                                                            (line 10150)
* character, escape, changing (ec):      Using Escape Sequences.
                                                            (line  5012)
* character, escape, while defining glyph: Using Symbols.   (line  7948)
* character, field delimiting (fc):      Fields.            (line  6692)
* character, field padding (fc):         Fields.            (line  6692)
* character, horizontal tab:             Tabs and Leaders.  (line  3785)
* character, hyphenation (\%):           Manipulating Hyphenation.
                                                            (line  5992)
* character, leader:                     Tabs and Leaders.  (line  3785)
* character, leader repetition (lc):     Leaders.           (line  6661)
* character, leader, and translations:   Character Translations.
                                                            (line  6751)
* character, leader, non-interpreted (\a): Leaders.         (line  6658)
* character, named (\C):                 Using Symbols.     (line  7793)
* character, newline, and translations:  Character Translations.
                                                            (line  6751)
* character, no-break control ('):       Requests and Macros.
                                                            (line  3813)
* character, no-break control, changing (c2): Control Characters.
                                                            (line  4738)
* character, ordinary:                   Identifiers.       (line  4577)
* character, soft hyphen, setting (shc): Manipulating Hyphenation.
                                                            (line  6032)
* character, special:                    Character Translations.
                                                            (line  6741)
* character, tab repetition (tc):        Tabs and Fields.   (line  6602)
* character, tab, and translations:      Character Translations.
                                                            (line  6751)
* character, tab, non-interpreted (\t):  Tabs and Fields.   (line  6485)
* character, transparent:                Using Symbols.     (line  7892)
* character, transparent dummy (\)):     Dummy Characters.  (line  8407)
* characters, end-of-sentence:           Using Symbols.     (line  7863)
* characters, end-of-sentence transparent: Sentences.       (line  3663)
* characters, hyphenation:               Using Symbols.     (line  7868)
* characters, input, and output glyphs, compatibility with AT&T troff: Other Differences.
                                                            (line 12609)
* characters, invalid for trf request:   I/O.               (line 11533)
* characters, invalid input:             Identifiers.       (line  4581)
* characters, overlapping:               Using Symbols.     (line  7882)
* characters, special:                   Sentences.         (line  3663)
* characters, special, list of (groff_char(7) man page): Using Symbols.
                                                            (line  7675)
* characters, unnamed, accessing with \N: Font Description File Format.
                                                            (line 13639)
* circle, filled, drawing (\D'C ...'):   Drawing Geometric Objects.
                                                            (line 10234)
* circle, outlined, drawing (\D'c ...'): Drawing Geometric Objects.
                                                            (line 10230)
* circle, solid, drawing (\D'C ...'):    Drawing Geometric Objects.
                                                            (line 10234)
* circle, stroked, drawing (\D'c ...'):  Drawing Geometric Objects.
                                                            (line 10230)
* class of characters (class):           Character Classes. (line  8034)
* classes, character:                    Character Classes. (line  8028)
* clearing input line trap (it, itc):    Input Line Traps.  (line 10680)
* closing brace escape sequence (\}):    Conditional Blocks.
                                                            (line  9250)
* closing file (close):                  I/O.               (line 11703)
* code page 1047 output encoding:        Groff Options.     (line   755)
* code page 1047, input encoding:        Input Encodings.   (line  3986)
* code, hyphenation (hcode):             Manipulating Hyphenation.
                                                            (line  6228)
* color name, background, register (.M): Colors.            (line  8765)
* color name, fill, register (.M):       Colors.            (line  8765)
* color name, stroke, register (.m):     Colors.            (line  8744)
* color, default:                        Colors.            (line  8720)
* color, fill:                           Colors.            (line  8678)
* color, stroke:                         Colors.            (line  8678)
* colors:                                Colors.            (line  8678)
* command prefix:                        Environment.       (line   835)
* command-line options:                  Groff Options.     (line   520)
* comments:                              Comments.          (line  5132)
* comments in device description files:  DESC File Format.  (line 13432)
* comments in font description files:    Font Description File Format.
                                                            (line 13592)
* comments, lining up with tabs:         Comments.          (line  5145)
* comments, with ds:                     Strings.           (line  8823)
* common features:                       Common Features.   (line  1201)
* common name space of macros, diversions, and strings: Identifiers.
                                                            (line  4668)
* comparison of strings:                 Operators in Conditionals.
                                                            (line  9108)
* comparison operators:                  Numeric Expressions.
                                                            (line  4416)
* compatibility mode:                    Warnings.          (line 12341)
* compatibility mode <1>:                Compatibility Mode.
                                                            (line 12404)
* compatibility mode, and parameters:    Gtroff Internals.  (line 12054)
* complementation, logical:              Numeric Expressions.
                                                            (line  4426)
* composite glyph names:                 Using Symbols.     (line  7688)
* conditional block, beginning (\{):     Conditional Blocks.
                                                            (line  9250)
* conditional block, end (\}):           Conditional Blocks.
                                                            (line  9250)
* conditional blocks:                    Conditional Blocks.
                                                            (line  9243)
* conditional expressions:               Operators in Conditionals.
                                                            (line  9027)
* conditional output for terminal (TTY): Operators in Conditionals.
                                                            (line  9053)
* conditional page break (ne):           Page Control.      (line  7195)
* conditionals and loops:                Conditionals and Loops.
                                                            (line  9020)
* configuring control characters:        Control Characters.
                                                            (line  4726)
* configuring the page length (pl):      Page Layout.       (line  7093)
* consecutive hyphenated lines (hlm):    Manipulating Hyphenation.
                                                            (line  6277)
* constant glyph space mode (cs):        Artificial Fonts.  (line  8241)
* contents, table of:                    Table of Contents. (line  1323)
* contents, table of <1>:                Leaders.           (line  6671)
* continuation, input line (\<RET>):     Line Continuation. (line  7019)
* continuation, output line (\c):        Line Continuation. (line  7048)
* continue request, in a while loop:     while.             (line  9393)
* continued output line register (.int): Line Continuation. (line  7073)
* continuous underlining (cu):           Artificial Fonts.  (line  8202)
* control character (.):                 Requests and Macros.
                                                            (line  3813)
* control character, changing (cc):      Control Characters.
                                                            (line  4738)
* control character, no-break ('):       Requests and Macros.
                                                            (line  3813)
* control character, no-break, changing (c2): Control Characters.
                                                            (line  4738)
* control characters:                    Control Characters.
                                                            (line  4726)
* control line:                          Requests and Macros.
                                                            (line  3825)
* control, line:                         Line Continuation. (line  7014)
* control, page:                         Page Control.      (line  7170)
* conventions for input:                 Input Conventions. (line  4051)
* conversion to basic units:             Measurements.      (line  4239)
* copy mode:                             Copy Mode.         (line  9699)
* copy mode <1>:                         Copy Mode.         (line  9699)
* copy mode, and cf request:             I/O.               (line 11521)
* copy mode, and device request:         Postprocessor Access.
                                                            (line 11744)
* copy mode, and length request:         Strings.           (line  8909)
* copy mode, and macro parameters:       Parameters.        (line  9616)
* copy mode, and output request:         Diversions.        (line 11106)
* copy mode, and trf request:            I/O.               (line 11521)
* copy mode, and write request:          I/O.               (line 11683)
* copy mode, and writec request:         I/O.               (line 11683)
* copy mode, and writem request:         I/O.               (line 11697)
* copy mode, and \!:                     Diversions.        (line 11074)
* copy mode, and \?:                     Operators in Conditionals.
                                                            (line  9116)
* copy mode, and \? <1>:                 Diversions.        (line 11074)
* copy mode, and \a:                     Leaders.           (line  6658)
* copy mode, and \t:                     Tabs and Fields.   (line  6485)
* copy mode, and \V:                     I/O.               (line 11721)
* copying environment (evc):             Environments.      (line 11362)
* correction between oblique and upright glyph (\/, \,): Italic Corrections.
                                                            (line  8328)
* correction between upright and oblique glyph (\/, \,): Italic Corrections.
                                                            (line  8338)
* correction, italic (\/):               Italic Corrections.
                                                            (line  8328)
* correction, left italic (\,):          Italic Corrections.
                                                            (line  8338)
* cover page in [ms], example markup:    ms Document Description Macros.
                                                            (line  2196)
* cp request, and glyph definitions:     Using Symbols.     (line  7948)
* cq glyph, at end of sentence:          Sentences.         (line  3663)
* cq glyph, at end of sentence <1>:      Using Symbols.     (line  7892)
* creating alias for register (aln):     Setting Registers. (line  5323)
* creating alias, for diversion (als):   Strings.           (line  8972)
* creating alias, for macro (als):       Strings.           (line  8972)
* creating alias, for string (als):      Strings.           (line  8972)
* creating new characters (char):        Using Symbols.     (line  7948)
* credits:                               Credits.           (line   444)
* cs request, and font styles:           Font Families.     (line  7505)
* cs request, and font translations:     Selecting Fonts.   (line  7406)
* cs request, incompatibilities with AT&T troff: Other Differences.
                                                            (line 12609)
* cs request, with fractional type sizes: Using Fractional Type Sizes.
                                                            (line  8604)
* CSTR #54 errata:                       Built-in Registers.
                                                            (line  5600)
* CSTR #54 errata <1>:                   Line Layout.       (line  6920)
* CSTR #54 errata <2>:                   Page Control.      (line  7185)
* CSTR #54 errata <3>:                   Artificial Fonts.  (line  8176)
* CSTR #54 errata <4>:                   Changing the Type Size.
                                                            (line  8475)
* CSTR #54 errata <5>:                   Page Motions.      (line 10040)
* CSTR #54 erratum, bp request:          Page Control.      (line  7185)
* CSTR #54 erratum, po request:          Line Layout.       (line  6920)
* CSTR #54 erratum, ps request:          Changing the Type Size.
                                                            (line  8475)
* CSTR #54 erratum, sb register:         Page Motions.      (line 10040)
* CSTR #54 erratum, st register:         Page Motions.      (line 10040)
* CSTR #54 erratum, yr register:         Built-in Registers.
                                                            (line  5600)
* CSTR #54 erratum, \S escape:           Artificial Fonts.  (line  8176)
* CSTR #54 erratum, \s escape sequence:  Changing the Type Size.
                                                            (line  8475)
* current directory:                     Macro Directories. (line   910)
* current input file name register (.F): Built-in Registers.
                                                            (line  5536)
* current page number (%):               Page Control.      (line  7188)
* current time, hours (hours):           Built-in Registers.
                                                            (line  5586)
* current time, minutes (minutes):       Built-in Registers.
                                                            (line  5583)
* current time, seconds (seconds):       Built-in Registers.
                                                            (line  5580)
* da request, and dn (dl):               Diversions.        (line 11026)
* da request, and warnings:              Warnings.          (line 12296)
* da request, and warnings <1>:          Warnings.          (line 12301)
* date, day of the month register (dy):  Built-in Registers.
                                                            (line  5592)
* date, day of the week register (dw):   Built-in Registers.
                                                            (line  5589)
* date, month of the year register (mo): Built-in Registers.
                                                            (line  5595)
* date, year register (year, yr):        Built-in Registers.
                                                            (line  5598)
* day of the month register (dy):        Built-in Registers.
                                                            (line  5592)
* day of the week register (dw):         Built-in Registers.
                                                            (line  5589)
* dd glyph, at end of sentence:          Sentences.         (line  3663)
* dd glyph, at end of sentence <1>:      Using Symbols.     (line  7892)
* de request, and while:                 while.             (line  9343)
* de, de1, dei requests, and warnings:   Warnings.          (line 12301)
* debugging:                             Debugging.         (line 12078)
* debugging page location traps:         Page Location Traps.
                                                            (line 10476)
* decimal point, as delimiter:           Delimiters.        (line  5090)
* decrementation, automatic, of a register: Auto-increment. (line  5364)
* default color:                         Colors.            (line  8720)
* default tab stops:                     Tabs and Fields.   (line  6493)
* default units:                         Default Units.     (line  4318)
* deferred output:                       Deferring Output.  (line 10337)
* defining character (char):             Using Symbols.     (line  7948)
* defining character class (class):      Character Classes. (line  8034)
* defining fallback character (fchar, fschar, schar): Using Symbols.
                                                            (line  7948)
* defining glyph (char):                 Using Symbols.     (line  7948)
* defining symbol (char):                Using Symbols.     (line  7948)
* delimited arguments, incompatibilities with AT&T troff: Compatibility Mode.
                                                            (line 12498)
* delimiters, for escape sequence arguments: Delimiters.    (line  5060)
* delimiting character, for fields (fc): Fields.            (line  6692)
* delimiting escape sequence arguments:  Delimiters.        (line  5060)
* depth, interpolation:                  Calling Macros.    (line  4929)
* depth, of last glyph (.cdp):           Environments.      (line 11388)
* DESC file format:                      DESC File Format.  (line 13425)
* DESC file, and font mounting:          Font Positions.    (line  7590)
* DESC file, and use_charnames_in_special keyword: Postprocessor Access.
                                                            (line 11760)
* description file, font:                Using Fonts.       (line  7295)
* device description files, comments:    DESC File Format.  (line 13432)
* device request, and copy mode:         Postprocessor Access.
                                                            (line 11744)
* device resolution:                     Page Geometry.     (line  4169)
* device resolution <1>:                 DESC File Format.  (line 13506)
* device resolution, obtaining in the formatter: Measurements.
                                                            (line  4240)
* devices for output:                    Output Device Intro.
                                                            (line   359)
* dg glyph, at end of sentence:          Sentences.         (line  3663)
* dg glyph, at end of sentence <1>:      Using Symbols.     (line  7892)
* di request, and warnings:              Warnings.          (line 12296)
* di request, and warnings <1>:          Warnings.          (line 12301)
* differences in implementation:         Implementation Differences.
                                                            (line 12388)
* digit-width space (\0):                Page Motions.      (line 10014)
* digits, as delimiters:                 Delimiters.        (line  5090)
* dimensions, line:                      Line Layout.       (line  6855)
* directories for fonts:                 Font Directories.  (line   934)
* directories for macros:                Macro Directories. (line   898)
* directory, current:                    Macro Directories. (line   910)
* directory, for tmac files:             Macro Directories. (line   900)
* directory, home:                       Macro Directories. (line   913)
* directory, platform-specific:          Macro Directories. (line   915)
* directory, site-local:                 Macro Directories. (line   915)
* directory, site-local <1>:             Font Directories.  (line   957)
* disabling hyphenation (\%):            Manipulating Hyphenation.
                                                            (line  5992)
* disabling \ (eo):                      Using Escape Sequences.
                                                            (line  5007)
* discardable horizontal space:          Manipulating Filling and Adjustment.
                                                            (line  5914)
* displays:                              Displays and Keeps.
                                                            (line  1290)
* displays [ms]:                         ms keeps and displays.
                                                            (line  2797)
* displays, and footnotes [ms]:          ms Footnotes.      (line  2958)
* distance to next vertical position trap register (.t): Page Location Traps.
                                                            (line 10520)
* diversion:                             Deferring Output.  (line 10337)
* diversion name register (.z):          Diversions.        (line 11000)
* diversion trap, setting (dt):          Diversion Traps.   (line 10668)
* diversion traps:                       Diversion Traps.   (line 10663)
* diversion, appending to (da, boxa):    Diversions.        (line 10944)
* diversion, beginning (di, box):        Diversions.        (line 10944)
* diversion, creating alias for (als):   Strings.           (line  8972)
* diversion, ending (di, box):           Diversions.        (line 10944)
* diversion, nested:                     Diversions.        (line 11000)
* diversion, removing (rm):              Strings.           (line  8967)
* diversion, removing alias for (rm):    Strings.           (line  9007)
* diversion, renaming (rn):              Strings.           (line  8964)
* diversion, stripping final newline:    Punning Names.     (line 11246)
* diversion, top-level:                  Diversions.        (line 10930)
* diversion, top-level, and bp:          Page Control.      (line  7183)
* diversion, top-level, and \!:          Diversions.        (line 11098)
* diversion, top-level, and \?:          Diversions.        (line 11103)
* diversion, unformatting (asciify):     Diversions.        (line 11120)
* diversion, vertical position in, register (.d): Diversions.
                                                            (line 11000)
* diversions:                            Diversions.        (line 10918)
* diversions <1>:                        Punning Names.     (line 11157)
* diversions, and traps:                 Page Location Traps.
                                                            (line 10618)
* diversions, shared name space with macros and strings: Identifiers.
                                                            (line  4668)
* division, truncating:                  Numeric Expressions.
                                                            (line  4359)
* dl register, and da (boxa):            Diversions.        (line 11026)
* dn register, and da (boxa):            Diversions.        (line 11026)
* document description macros, [ms]:     ms Document Description Macros.
                                                            (line  2132)
* document formats:                      Document Formats.  (line  1349)
* documents, multi-file:                 Debugging.         (line 12103)
* documents, structuring the source of:  Invoking Requests. (line  4803)
* dot, as delimiter:                     Delimiters.        (line  5090)
* double quote, embedding in a macro argument: Calling Macros.
                                                            (line  4875)
* double quotes, trailing, in strings:   Strings.           (line  8844)
* double-spacing (ls):                   Manipulating Spacing.
                                                            (line  6396)
* double-spacing (vs, pvs):              Changing the Vertical Spacing.
                                                            (line  8575)
* down-casing a string (stringdown):     Strings.           (line  8945)
* drawing a filled circle (\D'C ...'):   Drawing Geometric Objects.
                                                            (line 10234)
* drawing a filled ellipse (\D'E ...'):  Drawing Geometric Objects.
                                                            (line 10241)
* drawing a filled polygon (\D'P ...'):  Drawing Geometric Objects.
                                                            (line 10275)
* drawing a line (\D'l ...'):            Drawing Geometric Objects.
                                                            (line 10244)
* drawing a solid circle (\D'C ...'):    Drawing Geometric Objects.
                                                            (line 10234)
* drawing a solid ellipse (\D'E ...'):   Drawing Geometric Objects.
                                                            (line 10241)
* drawing a solid polygon (\D'P ...'):   Drawing Geometric Objects.
                                                            (line 10275)
* drawing a spline (\D'~ ...'):          Drawing Geometric Objects.
                                                            (line 10221)
* drawing a stroked circle (\D'c ...'):  Drawing Geometric Objects.
                                                            (line 10230)
* drawing a stroked ellipse (\D'e ...'): Drawing Geometric Objects.
                                                            (line 10237)
* drawing a stroked polygon (\D'p ...'): Drawing Geometric Objects.
                                                            (line 10269)
* drawing an arc (\D'a ...'):            Drawing Geometric Objects.
                                                            (line 10225)
* drawing an outlined circle (\D'c ...'): Drawing Geometric Objects.
                                                            (line 10230)
* drawing an outlined ellipse (\D'e ...'): Drawing Geometric Objects.
                                                            (line 10237)
* drawing an outlined polygon (\D'p ...'): Drawing Geometric Objects.
                                                            (line 10269)
* drawing horizontal lines (\l):         Drawing Geometric Objects.
                                                            (line 10142)
* drawing position:                      Page Geometry.     (line  4181)
* drawing position, vertical (nl):       Page Control.      (line  7240)
* drawing requests:                      Drawing Geometric Objects.
                                                            (line 10130)
* drawing vertical lines (\L):           Drawing Geometric Objects.
                                                            (line 10168)
* ds request, and comments:              Strings.           (line  8823)
* ds request, and double quotes:         Strings.           (line  8844)
* ds request, and leading spaces:        Strings.           (line  8844)
* ds, ds1 requests, and comments:        Comments.          (line  5139)
* ds, ds1 requests, and warnings:        Warnings.          (line 12301)
* dummy character (\&):                  Dummy Characters.  (line  8357)
* dummy character (\&), as control character suppressor: Requests and Macros.
                                                            (line  3820)
* dummy character (\&), effect on kerning: Ligatures and Kerning.
                                                            (line  8294)
* dummy character (\&), effect on \l escape sequence: Drawing Geometric Objects.
                                                            (line 10150)
* dummy character, transparent (\)):     Dummy Characters.  (line  8407)
* dummy environment, used by \w escape sequence: Page Motions.
                                                            (line 10034)
* dumping environments (pev):            Debugging.         (line 12150)
* dumping page location traps (ptr):     Debugging.         (line 12162)
* dumping registers (pnr):               Debugging.         (line 12158)
* dumping symbol table (pm):             Debugging.         (line 12154)
* EBCDIC output encoding:                Groff Options.     (line   755)
* EBCDIC, input encoding:                Input Encodings.   (line  3986)
* ejection, page:                        Page Geometry.     (line  4208)
* ejection, page <1>:                    Page Control.      (line  7170)
* ejection, page <2>:                    The Implicit Page Trap.
                                                            (line 10638)
* ejection, page, of final page:         End-of-input Traps.
                                                            (line 10834)
* ejection, page, prevented by vpt:      Vertical Position Traps.
                                                            (line 10414)
* el request, and warnings:              Warnings.          (line 12273)
* ellipse, filled, drawing (\D'E ...'):  Drawing Geometric Objects.
                                                            (line 10241)
* ellipse, outlined, drawing (\D'e ...'): Drawing Geometric Objects.
                                                            (line 10237)
* ellipse, solid, drawing (\D'E ...'):   Drawing Geometric Objects.
                                                            (line 10241)
* ellipse, stroked, drawing (\D'e ...'): Drawing Geometric Objects.
                                                            (line 10237)
* em glyph, and cflags:                  Using Symbols.     (line  7875)
* em scaling unit (m):                   Measurements.      (line  4277)
* embolding of special fonts:            Artificial Fonts.  (line  8230)
* empty line:                            Breaking.          (line  3749)
* en scaling unit (n):                   Measurements.      (line  4281)
* enabling vertical position traps (vpt): Vertical Position Traps.
                                                            (line 10407)
* encoding, input, code page 1047:       Input Encodings.   (line  3986)
* encoding, input, EBCDIC:               Input Encodings.   (line  3986)
* encoding, input, Latin-1 (ISO 8859-1): Input Encodings.   (line  3991)
* encoding, input, Latin-2 (ISO 8859-2): Input Encodings.   (line  4004)
* encoding, input, Latin-5 (ISO 8859-9): Input Encodings.   (line  4010)
* encoding, input, Latin-9 (ISO 8859-15): Input Encodings.  (line  4015)
* encoding, output, ASCII:               Groff Options.     (line   743)
* encoding, output, code page 1047:      Groff Options.     (line   755)
* encoding, output, EBCDIC:              Groff Options.     (line   755)
* encoding, output, ISO 646:             Groff Options.     (line   743)
* encoding, output, Latin-1 (ISO 8859-1): Groff Options.    (line   747)
* encoding, output, UTF-8:               Groff Options.     (line   751)
* end of conditional block (\}):         Conditional Blocks.
                                                            (line  9250)
* end-of-input macro (em):               End-of-input Traps.
                                                            (line 10807)
* end-of-input trap, setting (em):       End-of-input Traps.
                                                            (line 10807)
* end-of-input traps:                    End-of-input Traps.
                                                            (line 10806)
* end-of-sentence characters:            Sentences.         (line  3619)
* end-of-sentence characters <1>:        Using Symbols.     (line  7863)
* end-of-sentence transparent characters: Sentences.        (line  3663)
* ending diversion (di, box):            Diversions.        (line 10944)
* endnotes:                              Footnotes and Endnotes.
                                                            (line  1313)
* environment:                           Deferring Output.  (line 10337)
* environment availability and naming, incompatibilities with: Other Differences.
                                                            (line 12583)
* environment number/name register (.ev): Environments.     (line 11309)
* environment variables:                 Environment.       (line   827)
* environment, copying (evc):            Environments.      (line 11362)
* environment, dimensions of last glyph (.w, .cht, .cdp, .csk): Environments.
                                                            (line 11388)
* environment, dummy, used by \w escape sequence: Page Motions.
                                                            (line 10034)
* environment, previous line length (.n): Environments.     (line 11403)
* environment, switching (ev):           Environments.      (line 11309)
* environments:                          Environments.      (line 11268)
* environments, dumping (pev):           Debugging.         (line 12150)
* equality operator:                     Numeric Expressions.
                                                            (line  4416)
* equation example [ms]:                 ms Insertions.     (line  2903)
* equations [ms]:                        ms Insertions.     (line  2844)
* escape character, changing (ec):       Using Escape Sequences.
                                                            (line  5012)
* escape character, formatting (\e):     Using Escape Sequences.
                                                            (line  4999)
* escape character, while defining glyph: Using Symbols.    (line  7948)
* escape sequence:                       Formatter Instructions.
                                                            (line  4714)
* escape sequence argument delimiters:   Delimiters.        (line  5060)
* escape sequences:                      Using Escape Sequences.
                                                            (line  4942)
* escape sequences, brace (\{, \}):      Conditional Blocks.
                                                            (line  9250)
* escaping newline characters, in strings: Strings.         (line  8853)
* ex request, use in debugging:          Debugging.         (line 12132)
* ex request, used with nx and rd:       I/O.               (line 11591)
* example markup, bulleted list [ms]:    Lists in ms.       (line  2590)
* example markup, cover page in [ms]:    ms Document Description Macros.
                                                            (line  2196)
* example markup, glossary-style list [ms]: Lists in ms.    (line  2637)
* example markup, numbered list [ms]:    Lists in ms.       (line  2609)
* examples of invocation:                Invocation Examples.
                                                            (line  1002)
* exiting (ex):                          Debugging.         (line 12132)
* expansion of strings (\*):             Strings.           (line  8802)
* explicit hyphen (\%):                  Manipulating Hyphenation.
                                                            (line  6277)
* explicit hyphenation:                  Manipulating Hyphenation.
                                                            (line  5948)
* expression, limitation of logical not in: Numeric Expressions.
                                                            (line  4426)
* expression, order of evaluation:       Numeric Expressions.
                                                            (line  4446)
* expressions, and register format:      Assigning Register Formats.
                                                            (line  5484)
* expressions, and space characters:     Numeric Expressions.
                                                            (line  4538)
* expressions, conditional:              Operators in Conditionals.
                                                            (line  9027)
* expressions, numeric:                  Numeric Expressions.
                                                            (line  4348)
* extra post-vertical line space (\x):   Changing the Vertical Spacing.
                                                            (line  8568)
* extra post-vertical line space register (.a): Manipulating Spacing.
                                                            (line  6420)
* extra pre-vertical line space (\x):    Changing the Vertical Spacing.
                                                            (line  8560)
* extra spaces between words:            Adjustment.        (line  3775)
* extreme values representable with Roman numerals: Assigning Register Formats.
                                                            (line  5465)
* extremum operators (>?, <?):           Numeric Expressions.
                                                            (line  4407)
* f scaling unit:                        Colors.            (line  8712)
* factor, zoom, of a font (fzoom):       Selecting Fonts.   (line  7421)
* fallback character, defining (fchar, fschar, schar): Using Symbols.
                                                            (line  7948)
* fallback glyph, removing definition (rchar, rfschar): Using Symbols.
                                                            (line  8005)
* fam request, and changing fonts:       Selecting Fonts.   (line  7352)
* families, font:                        Font Families.     (line  7448)
* family, font:                          Using Fonts.       (line  7280)
* features, common:                      Common Features.   (line  1201)
* fi request, causing implicit break:    Manipulating Filling and Adjustment.
                                                            (line  5624)
* field delimiting character (fc):       Fields.            (line  6692)
* field padding character (fc):          Fields.            (line  6692)
* fields:                                Fields.            (line  6692)
* fields, and tabs:                      Tabs and Fields.   (line  6481)
* figure space (\0):                     Page Motions.      (line 10014)
* figures [ms]:                          ms Insertions.     (line  2844)
* file formats:                          File Formats.      (line 12674)
* file names, breaking (\:):             Manipulating Hyphenation.
                                                            (line  6008)
* file, appending to (opena):            I/O.               (line 11671)
* file, closing (close):                 I/O.               (line 11703)
* file, font description:                Using Fonts.       (line  7295)
* file, inclusion (so):                  I/O.               (line 11473)
* file, macro, search path:              Macro Directories. (line   900)
* file, opening (open):                  I/O.               (line 11671)
* file, processing next (nx):            I/O.               (line 11553)
* file, writing to (write, writec):      I/O.               (line 11683)
* files, font:                           Device and Font Description Files.
                                                            (line 13401)
* fill color:                            Colors.            (line  8678)
* fill color name register (.M):         Colors.            (line  8765)
* fill mode (fi), enabling:              Manipulating Filling and Adjustment.
                                                            (line  5685)
* fill mode, and \c:                     Line Continuation. (line  7054)
* fill mode, disabling:                  Manipulating Filling and Adjustment.
                                                            (line  5692)
* filled circle, drawing (\D'C ...'):    Drawing Geometric Objects.
                                                            (line 10234)
* filled ellipse, drawing (\D'E ...'):   Drawing Geometric Objects.
                                                            (line 10241)
* filled polygon, drawing (\D'P ...'):   Drawing Geometric Objects.
                                                            (line 10275)
* filling:                               Filling.           (line  3581)
* filling and adjustment, manipulating:  Manipulating Filling and Adjustment.
                                                            (line  5624)
* filling of output, disabling (nf):     Manipulating Filling and Adjustment.
                                                            (line  5692)
* filling of output, enabling (fi):      Manipulating Filling and Adjustment.
                                                            (line  5685)
* filling, and break warnings:           Warnings.          (line 12262)
* filling, and inter-sentence space:     Manipulating Filling and Adjustment.
                                                            (line  5898)
* final newline, stripping in diversions: Punning Names.    (line 11246)
* fl request, causing implicit break:    Manipulating Filling and Adjustment.
                                                            (line  5624)
* floating keep:                         Displays and Keeps.
                                                            (line  1301)
* flush output (fl):                     Debugging.         (line 12167)
* font:                                  Using Fonts.       (line  7280)
* font aliasing with third argument to fp request: Font Positions.
                                                            (line  7561)
* font description file:                 Using Fonts.       (line  7295)
* font description file format:          DESC File Format.  (line 13425)
* font description file, format:         Font Description File Format.
                                                            (line 13572)
* font description files, comments:      Font Description File Format.
                                                            (line 13592)
* font directories:                      Font Directories.  (line   934)
* font families:                         Font Families.     (line  7448)
* font family:                           Using Fonts.       (line  7280)
* font family, changing (fam, \F):       Font Families.     (line  7467)
* font file, format:                     Font Description File Format.
                                                            (line 13572)
* font files:                            Device and Font Description Files.
                                                            (line 13401)
* font for underlining (uf):             Artificial Fonts.  (line  8206)
* font height, changing (\H):            Artificial Fonts.  (line  8132)
* font metrics:                          Using Fonts.       (line  7295)
* font mounting, automatic:              Selecting Fonts.   (line  7365)
* font path:                             Font Directories.  (line   942)
* font position register (.f):           Font Positions.    (line  7578)
* font positions:                        Font Positions.    (line  7540)
* font slant, changing (\S):             Artificial Fonts.  (line  8161)
* font style:                            Using Fonts.       (line  7280)
* font style, abstract:                  Using Fonts.       (line  7302)
* font style, abstract, setting up (sty): Font Families.    (line  7505)
* font styles:                           Font Families.     (line  7448)
* font translation (ftr):                Selecting Fonts.   (line  7406)
* font, magnification (fzoom):           Selecting Fonts.   (line  7421)
* font, mounting (fp):                   Font Positions.    (line  7552)
* font, optical size:                    Selecting Fonts.   (line  7421)
* font, previous, selecting (\f[], \fP): Selecting Fonts.   (line  7378)
* font, previous, slecting (ft):         Selecting Fonts.   (line  7352)
* font, selection:                       Selecting Fonts.   (line  7343)
* font, special:                         Using Fonts.       (line  7280)
* font, text:                            Using Fonts.       (line  7280)
* font, unstyled:                        Using Fonts.       (line  7280)
* font, zoom factor (fzoom):             Selecting Fonts.   (line  7421)
* fonts, artificial:                     Artificial Fonts.  (line  8122)
* fonts, changing (ft, \f):              Selecting Fonts.   (line  7352)
* fonts, searching:                      Font Directories.  (line   934)
* fonts, special:                        Special Fonts.     (line  8092)
* footers:                               Page Layout.       (line  7115)
* footers <1>:                           Page Location Traps.
                                                            (line 10440)
* footers [ms]:                          ms Headers and Footers.
                                                            (line  3052)
* footnote marker [ms]:                  ms Footnotes.      (line  2920)
* footnotes:                             Footnotes and Endnotes.
                                                            (line  1313)
* footnotes [ms]:                        ms Footnotes.      (line  2920)
* footnotes, and displays [ms]:          ms Footnotes.      (line  2958)
* footnotes, and keeps [ms]:             ms Footnotes.      (line  2958)
* form letters:                          I/O.               (line 11575)
* format of font description file:       DESC File Format.  (line 13425)
* format of font description files:      Font Description File Format.
                                                            (line 13572)
* format of font files:                  Font Description File Format.
                                                            (line 13572)
* format of register (\g):               Assigning Register Formats.
                                                            (line  5479)
* format, paper:                         Paper Format.      (line   972)
* formats, file:                         File Formats.      (line 12674)
* formatter instructions:                Formatter Instructions.
                                                            (line  4703)
* formatting a backslash glyph (\[rs]):  Using Escape Sequences.
                                                            (line  5002)
* formatting a title line (tl):          Page Layout.       (line  7120)
* formatting the escape character (\e):  Using Escape Sequences.
                                                            (line  4999)
* formatting the time:                   I/O.               (line 11656)
* fp request, and font translations:     Selecting Fonts.   (line  7406)
* fp request, incompatibilities with AT&T troff: Other Differences.
                                                            (line 12609)
* fractional point sizes:                Using Fractional Type Sizes.
                                                            (line  8597)
* fractional point sizes <1>:            Other Differences. (line 12587)
* fractional type sizes:                 Using Fractional Type Sizes.
                                                            (line  8597)
* fractional type sizes <1>:             Other Differences. (line 12587)
* fractional type sizes in ms macros:    Differences from AT&T ms.
                                                            (line  3315)
* French spacing:                        Sentences.         (line  3619)
* fspecial request, and font styles:     Font Families.     (line  7505)
* fspecial request, and font translations: Selecting Fonts. (line  7406)
* fspecial request, and glyph search order: Using Symbols.  (line  7612)
* fspecial request, and imitating bold:  Artificial Fonts.  (line  8230)
* ft request, and font translations:     Selecting Fonts.   (line  7406)
* full-service macro package:            Major Macro Packages.
                                                            (line  1397)
* geometry, page:                        Page Geometry.     (line  4162)
* GGL (groff glyph list):                Using Symbols.     (line  7688)
* GGL (groff glyph list) <1>:            Character Classes. (line  8055)
* glossary-style list, example markup [ms]: Lists in ms.    (line  2637)
* glyph:                                 Using Symbols.     (line  7600)
* glyph for line drawing:                Drawing Geometric Objects.
                                                            (line 10168)
* glyph names, composite:                Using Symbols.     (line  7688)
* glyph pile (\b):                       Drawing Geometric Objects.
                                                            (line 10308)
* glyph properties (cflags):             Using Symbols.     (line  7846)
* glyph, box rule (\[br]):               Drawing Geometric Objects.
                                                            (line 10168)
* glyph, constant space:                 Artificial Fonts.  (line  8241)
* glyph, defining (char):                Using Symbols.     (line  7948)
* glyph, distinguished from character:   Using Symbols.     (line  7600)
* glyph, for line drawing:               Drawing Geometric Objects.
                                                            (line 10146)
* glyph, for margins (mc):               Miscellaneous.     (line 11897)
* glyph, last, dimensions (.w, .cht, .cdp, .csk): Environments.
                                                            (line 11388)
* glyph, leader repetition (lc):         Leaders.           (line  6661)
* glyph, numbered (\N):                  Character Translations.
                                                            (line  6741)
* glyph, numbered (\N) <1>:              Using Symbols.     (line  7810)
* glyph, removing definition (rchar, rfschar): Using Symbols.
                                                            (line  8005)
* glyph, soft hyphen (hy):               Manipulating Hyphenation.
                                                            (line  6032)
* glyph, tab repetition (tc):            Tabs and Fields.   (line  6602)
* glyph, underscore (\[ru]):             Drawing Geometric Objects.
                                                            (line 10146)
* glyphs, available, list of (groff_char(7) man page): Using Symbols.
                                                            (line  7675)
* glyphs, output, and input characters, compatibility with AT&T troff: Other Differences.
                                                            (line 12609)
* glyphs, overstriking (\o):             Page Motions.      (line 10097)
* glyphs, unnamed:                       Using Symbols.     (line  7820)
* glyphs, unnamed, accessing with \N:    Font Description File Format.
                                                            (line 13639)
* GNU troff, identification register (.g): Built-in Registers.
                                                            (line  5539)
* GNU troff, PID register ($$):          Built-in Registers.
                                                            (line  5571)
* GNU troff, process ID register ($$):   Built-in Registers.
                                                            (line  5571)
* GNU-specific register (.g):            Built-in Registers.
                                                            (line  5539)
* graphic renditions:                    Using Fonts.       (line  7323)
* greater than (or equal to) operator:   Numeric Expressions.
                                                            (line  4416)
* groff capabilities:                    groff Capabilities.
                                                            (line   281)
* groff glyph list (GGL):                Using Symbols.     (line  7688)
* groff glyph list (GGL) <1>:            Character Classes. (line  8055)
* groff invocation:                      Invoking groff.    (line   454)
* groff, and pi request:                 I/O.               (line 11629)
* groff--what is it?:                    What Is groff?.    (line   264)
* GROFF_BIN_PATH, environment variable:  Environment.       (line   831)
* GROFF_COMMAND_PREFIX, environment variable: Environment.  (line   835)
* GROFF_ENCODING, environment variable:  Environment.       (line   846)
* GROFF_FONT_PATH, environment variable: Environment.       (line   855)
* GROFF_FONT_PATH, environment variable <1>: Font Directories.
                                                            (line   955)
* GROFF_TMAC_PATH, environment variable: Environment.       (line   862)
* GROFF_TMAC_PATH, environment variable <1>: Macro Directories.
                                                            (line   908)
* GROFF_TMPDIR, environment variable:    Environment.       (line   869)
* GROFF_TYPESETTER, environment variable: Environment.      (line   877)
* grohtml, the program:                  Groff Options.     (line   766)
* gtroff, interactive use:               Debugging.         (line 12167)
* gtroff, output:                        gtroff Output.     (line 12680)
* gtroff, reference:                     GNU troff Reference.
                                                            (line  3549)
* hair space (\^):                       Page Motions.      (line 10008)
* hcode request, and glyph definitions:  Using Symbols.     (line  7948)
* headers:                               Page Layout.       (line  7115)
* headers <1>:                           Page Location Traps.
                                                            (line 10440)
* headers [ms]:                          ms Headers and Footers.
                                                            (line  3052)
* height, font, changing (\H):           Artificial Fonts.  (line  8132)
* height, of last glyph (.cht):          Environments.      (line 11388)
* high-water mark register (.h):         Diversions.        (line 11007)
* home directory:                        Macro Directories. (line   913)
* horizontal discardable space:          Manipulating Filling and Adjustment.
                                                            (line  5914)
* horizontal input line position register (hp): Page Motions.
                                                            (line 10090)
* horizontal input line position, saving (\k): Page Motions.
                                                            (line 10084)
* horizontal line, drawing (\l):         Drawing Geometric Objects.
                                                            (line 10142)
* horizontal motion (\h):                Page Motions.      (line  9982)
* horizontal motion quantum:             DESC File Format.  (line 13445)
* horizontal motion quantum register (.H): Motion Quanta.   (line  4299)
* horizontal output line position register (.k): Page Motions.
                                                            (line 10093)
* horizontal resolution:                 DESC File Format.  (line 13445)
* horizontal resolution register (.H):   Motion Quanta.     (line  4299)
* horizontal space (\h):                 Page Motions.      (line  9982)
* horizontal space, unformatting:        Punning Names.     (line 11246)
* horizontal tab character:              Tabs and Leaders.  (line  3785)
* hours, current time (hours):           Built-in Registers.
                                                            (line  5586)
* hpf request, and hyphenation language: Manipulating Hyphenation.
                                                            (line  6263)
* hw request, and hy restrictions:       Manipulating Hyphenation.
                                                            (line  5970)
* hw request, and hyphenation language:  Manipulating Hyphenation.
                                                            (line  6263)
* hy glyph, and cflags:                  Using Symbols.     (line  7875)
* hyphen, explicit (\%):                 Manipulating Hyphenation.
                                                            (line  6277)
* hyphenated lines, consecutive (hlm):   Manipulating Hyphenation.
                                                            (line  6277)
* hyphenating characters:                Using Symbols.     (line  7868)
* hyphenation:                           Hyphenation.       (line  3707)
* hyphenation character (\%):            Manipulating Hyphenation.
                                                            (line  5992)
* hyphenation code (hcode):              Manipulating Hyphenation.
                                                            (line  6228)
* hyphenation consecutive line count register (.hlc): Manipulating Hyphenation.
                                                            (line  6284)
* hyphenation consecutive line limit register (.hlm): Manipulating Hyphenation.
                                                            (line  6284)
* hyphenation exceptions:                Manipulating Hyphenation.
                                                            (line  5956)
* hyphenation language register (.hla):  Manipulating Hyphenation.
                                                            (line  6270)
* hyphenation margin (hym):              Manipulating Hyphenation.
                                                            (line  6290)
* hyphenation margin register (.hym):    Manipulating Hyphenation.
                                                            (line  6300)
* hyphenation mode register (.hy):       Manipulating Hyphenation.
                                                            (line  6054)
* hyphenation parameters, automatic:     Manipulating Hyphenation.
                                                            (line  6042)
* hyphenation pattern files:             Manipulating Hyphenation.
                                                            (line  6116)
* hyphenation patterns (hpf):            Manipulating Hyphenation.
                                                            (line  6170)
* hyphenation space (hys):               Manipulating Hyphenation.
                                                            (line  6305)
* hyphenation space adjustment threshold: Manipulating Hyphenation.
                                                            (line  6305)
* hyphenation space adjustment threshold register (.hys): Manipulating Hyphenation.
                                                            (line  6316)
* hyphenation, automatic:                Manipulating Hyphenation.
                                                            (line  5941)
* hyphenation, disabling (\%):           Manipulating Hyphenation.
                                                            (line  5992)
* hyphenation, explicit:                 Manipulating Hyphenation.
                                                            (line  5948)
* hyphenation, incompatibilities with AT&T troff: Other Differences.
                                                            (line 12557)
* hyphenation, manipulating:             Manipulating Hyphenation.
                                                            (line  5941)
* hyphenation, manual:                   Manipulating Hyphenation.
                                                            (line  5948)
* i scaling unit:                        Measurements.      (line  4249)
* i/o:                                   I/O.               (line 11469)
* IBM code page 1047 input encoding:     Input Encodings.   (line  3986)
* IBM code page 1047 output encoding:    Groff Options.     (line   755)
* identifiers:                           Identifiers.       (line  4574)
* identifiers, undefined:                Identifiers.       (line  4656)
* ie request, and font translations:     Selecting Fonts.   (line  7406)
* ie request, and warnings:              Warnings.          (line 12273)
* ie request, operators to use with:     Operators in Conditionals.
                                                            (line  9027)
* if request, and font translations:     Selecting Fonts.   (line  7406)
* if request, and the ! operator:        Numeric Expressions.
                                                            (line  4365)
* if request, operators to use with:     Operators in Conditionals.
                                                            (line  9027)
* if-else:                               if-else.           (line  9211)
* if-then:                               if-then.           (line  9170)
* imitating boldface (bd):               Artificial Fonts.  (line  8213)
* implementation differences:            Implementation Differences.
                                                            (line 12388)
* implicit line break:                   Breaking.          (line  3725)
* implicit trap:                         The Implicit Page Trap.
                                                            (line 10638)
* in request, causing implicit break:    Manipulating Filling and Adjustment.
                                                            (line  5624)
* in request, using + and - with:        Numeric Expressions.
                                                            (line  4463)
* inch scaling unit (i):                 Measurements.      (line  4249)
* including a file (so):                 I/O.               (line 11473)
* incompatibilities with AT&T troff:     Implementation Differences.
                                                            (line 12388)
* increment value without changing the register: Auto-increment.
                                                            (line  5403)
* incrementation, automatic, of a register: Auto-increment. (line  5364)
* indentation (in):                      Line Layout.       (line  6874)
* indentation, of roff source code:      Invoking Requests. (line  4803)
* index, in macro package:               Indexing.          (line  1340)
* indicator, scaling:                    Measurements.      (line  4232)
* indirect assignments:                  Interpolating Registers.
                                                            (line  5340)
* input and output requests:             I/O.               (line 11469)
* input characters and output glyphs, compatibility with AT&T troff: Other Differences.
                                                            (line 12609)
* input characters, invalid:             Identifiers.       (line  4581)
* input conventions:                     Input Conventions. (line  4051)
* input encoding, code page 1047:        Input Encodings.   (line  3986)
* input encoding, EBCDIC:                Input Encodings.   (line  3986)
* input encoding, Latin-1 (ISO 8859-1):  Input Encodings.   (line  3991)
* input encoding, Latin-2 (ISO 8859-2):  Input Encodings.   (line  4004)
* input encoding, Latin-5 (ISO 8859-9):  Input Encodings.   (line  4010)
* input encoding, Latin-9 (ISO 8859-15): Input Encodings.   (line  4015)
* input file name, current, register (.F): Built-in Registers.
                                                            (line  5536)
* input level:                           Calling Macros.    (line  4929)
* input level in delimited arguments:    Compatibility Mode.
                                                            (line 12498)
* input line continuation (\<RET>):      Line Continuation. (line  7019)
* input line number register (.c, c.):   Built-in Registers.
                                                            (line  5532)
* input line number, setting (lf):       Debugging.         (line 12103)
* input line position, horizontal, saving (\k): Page Motions.
                                                            (line 10084)
* input line trap, clearing (it, itc):   Input Line Traps.  (line 10680)
* input line trap, setting (it, itc):    Input Line Traps.  (line 10680)
* input line traps:                      Input Line Traps.  (line 10678)
* input line traps and interrupted lines (itc): Input Line Traps.
                                                            (line 10704)
* input line, horizontal position, register (hp): Page Motions.
                                                            (line 10090)
* input line, productive:                Manipulating Filling and Adjustment.
                                                            (line  5797)
* input stack, backtrace (backtrace):    Debugging.         (line 12176)
* input stack, setting limit:            Debugging.         (line 12197)
* input token:                           Gtroff Internals.  (line 11970)
* input, 8-bit:                          Font Description File Format.
                                                            (line 13639)
* input, standard, reading from (rd):    I/O.               (line 11558)
* inserting horizontal space (\h):       Page Motions.      (line  9982)
* installation:                          Installation.      (line   370)
* instructing the formatter:             Formatter Instructions.
                                                            (line  4703)
* inter-sentence space size register (.sss): Manipulating Filling and Adjustment.
                                                            (line  5887)
* inter-sentence space, additional:      Manipulating Filling and Adjustment.
                                                            (line  5893)
* inter-word spacing, minimal:           Manipulating Filling and Adjustment.
                                                            (line  5891)
* interactive use of gtroff:             Debugging.         (line 12167)
* intercepting requests:                 Control Characters.
                                                            (line  4765)
* intermediate output:                   gtroff Output.     (line 12690)
* interpolating registers (\n):          Interpolating Registers.
                                                            (line  5335)
* interpolation:                         Requests and Macros.
                                                            (line  3840)
* interpolation depth:                   Calling Macros.    (line  4929)
* interpolation depth in delimited arguments: Compatibility Mode.
                                                            (line 12498)
* interpolation of strings (\*):         Strings.           (line  8802)
* interpretation mode:                   Copy Mode.         (line  9708)
* interrupted line:                      Line Continuation. (line  7048)
* interrupted line register (.int):      Line Continuation. (line  7073)
* interrupted lines and input line traps (itc): Input Line Traps.
                                                            (line 10704)
* introduction:                          Introduction.      (line   222)
* invalid characters for trf request:    I/O.               (line 11533)
* invalid input characters:              Identifiers.       (line  4581)
* invocation examples:                   Invocation Examples.
                                                            (line  1002)
* invoking groff:                        Invoking groff.    (line   454)
* invoking requests:                     Invoking Requests. (line  4781)
* ISO 646 output encoding:               Groff Options.     (line   743)
* ISO 8859-1 (Latin-1) output encoding:  Groff Options.     (line   747)
* ISO 8859-1 (Latin-1), input encoding:  Input Encodings.   (line  3991)
* ISO 8859-15 (Latin-9), input encoding: Input Encodings.   (line  4015)
* ISO 8859-2 (Latin-2), input encoding:  Input Encodings.   (line  4004)
* ISO 8859-9 (Latin-5), input encoding:  Input Encodings.   (line  4010)
* italic correction (\/):                Italic Corrections.
                                                            (line  8328)
* justifying text:                       Manipulating Filling and Adjustment.
                                                            (line  5624)
* justifying text (rj):                  Manipulating Filling and Adjustment.
                                                            (line  5866)
* keep, floating:                        Displays and Keeps.
                                                            (line  1301)
* keeps (introduction):                  Displays and Keeps.
                                                            (line  1297)
* keeps [ms]:                            ms keeps and displays.
                                                            (line  2743)
* keeps, and footnotes [ms]:             ms Footnotes.      (line  2958)
* kerning and ligatures:                 Ligatures and Kerning.
                                                            (line  8253)
* kerning enabled register (.kern):      Ligatures and Kerning.
                                                            (line  8289)
* kerning, activating (kern):            Ligatures and Kerning.
                                                            (line  8289)
* kerning, track:                        Ligatures and Kerning.
                                                            (line  8300)
* landscape page orientation:            Paper Format.      (line   972)
* language [ms]:                         ms language and localization.
                                                            (line  3003)
* last glyph, dimensions (.w, .cht, .cdp, .csk): Environments.
                                                            (line 11388)
* last-requested point size registers (.psr, .sr): Using Fractional Type Sizes.
                                                            (line  8638)
* last-requested type size registers (.psr, .sr): Using Fractional Type Sizes.
                                                            (line  8638)
* Latin-1 (ISO 8859-1) output encoding:  Groff Options.     (line   747)
* Latin-1 (ISO 8859-1), input encoding:  Input Encodings.   (line  3991)
* Latin-2 (ISO 8859-2), input encoding:  Input Encodings.   (line  4004)
* Latin-5 (ISO 8859-9), input encoding:  Input Encodings.   (line  4010)
* Latin-9 (ISO 8859-15), input encoding: Input Encodings.   (line  4015)
* layout, line:                          Line Layout.       (line  6855)
* layout, page:                          Page Layout.       (line  7087)
* lc request, and glyph definitions:     Using Symbols.     (line  7948)
* leader character:                      Tabs and Leaders.  (line  3785)
* leader character <1>:                  Leaders.           (line  6652)
* leader character, and translations:    Character Translations.
                                                            (line  6751)
* leader character, non-interpreted (\a): Leaders.          (line  6658)
* leader repetition character (lc):      Leaders.           (line  6661)
* leaders:                               Leaders.           (line  6645)
* leading:                               Manipulating Type Size and Vertical Spacing.
                                                            (line  8444)
* leading space macro (lsm):             Breaking.          (line  3757)
* leading space traps:                   Leading Space Traps.
                                                            (line 10788)
* leading spaces:                        Breaking.          (line  3757)
* leading spaces macro (lsm):            Leading Space Traps.
                                                            (line 10791)
* leading spaces with ds:                Strings.           (line  8844)
* left italic correction (\,):           Italic Corrections.
                                                            (line  8338)
* left margin (po):                      Line Layout.       (line  6870)
* length of a string (length):           Strings.           (line  8909)
* length of line (ll):                   Line Layout.       (line  6878)
* length of previous line (.n):          Environments.      (line 11403)
* length of the page, configuring (pl):  Page Layout.       (line  7093)
* length of title line, configuring (lt): Page Layout.      (line  7137)
* length request, and copy mode:         Strings.           (line  8909)
* less than (or equal to) operator:      Numeric Expressions.
                                                            (line  4416)
* letters, form:                         I/O.               (line 11575)
* level, input:                          Calling Macros.    (line  4929)
* level, suppression nesting, register:  Suppressing Output.
                                                            (line 11463)
* lf request, incompatibilities with AT&T troff: Other Differences.
                                                            (line 12579)
* ligature:                              Using Symbols.     (line  7600)
* ligatures and kerning:                 Ligatures and Kerning.
                                                            (line  8253)
* ligatures enabled register (.lg):      Ligatures and Kerning.
                                                            (line  8271)
* ligatures, activating (lg):            Ligatures and Kerning.
                                                            (line  8271)
* limitations of \b escape sequence:     Drawing Geometric Objects.
                                                            (line 10316)
* line break:                            Manipulating Filling and Adjustment.
                                                            (line  5624)
* line break (introduction):             Basics.            (line  1188)
* line break, output:                    Breaking.          (line  3725)
* line control:                          Line Continuation. (line  7014)
* line dimensions:                       Line Layout.       (line  6855)
* line drawing glyph:                    Drawing Geometric Objects.
                                                            (line 10146)
* line drawing glyph <1>:                Drawing Geometric Objects.
                                                            (line 10168)
* line indentation (in):                 Line Layout.       (line  6874)
* line layout:                           Line Layout.       (line  6855)
* line length (ll):                      Line Layout.       (line  6878)
* line length register (.l):             Line Layout.       (line  7003)
* line length, previous (.n):            Environments.      (line 11403)
* line number, input, register (.c, c.): Built-in Registers.
                                                            (line  5532)
* line number, output, register (ln):    Miscellaneous.     (line 11820)
* line numbers, printing (nm):           Miscellaneous.     (line 11798)
* line space, extra post-vertical (\x):  Changing the Vertical Spacing.
                                                            (line  8568)
* line space, extra pre-vertical (\x):   Changing the Vertical Spacing.
                                                            (line  8560)
* line spacing register (.L):            Manipulating Spacing.
                                                            (line  6400)
* line spacing, post-vertical (pvs):     Changing the Vertical Spacing.
                                                            (line  8572)
* line thickness (\D't ...'):            Drawing Geometric Objects.
                                                            (line 10298)
* line, blank:                           Breaking.          (line  3749)
* line, drawing (\D'l ...'):             Drawing Geometric Objects.
                                                            (line 10244)
* line, horizontal, drawing (\l):        Drawing Geometric Objects.
                                                            (line 10142)
* line, input, continuation (\<RET>):    Line Continuation. (line  7019)
* line, input, horizontal position, register (hp): Page Motions.
                                                            (line 10090)
* line, input, horizontal position, saving (\k): Page Motions.
                                                            (line 10084)
* line, interrupted:                     Line Continuation. (line  7048)
* line, output, continuation (\c):       Line Continuation. (line  7048)
* line, output, horizontal position, register (.k): Page Motions.
                                                            (line 10093)
* line, productive input:                Manipulating Filling and Adjustment.
                                                            (line  5797)
* line, vertical, drawing (\L):          Drawing Geometric Objects.
                                                            (line 10168)
* line-tabs mode:                        Tabs and Fields.   (line  6615)
* lines, blank, disabling:               Manipulating Spacing.
                                                            (line  6456)
* lines, centering (ce):                 Manipulating Filling and Adjustment.
                                                            (line  5827)
* lines, centering (introduction):       Basics.            (line  1170)
* lines, consecutive hyphenated (hlm):   Manipulating Hyphenation.
                                                            (line  6277)
* lines, interrupted, and input line traps (itc): Input Line Traps.
                                                            (line 10704)
* lines, right-aligning (introduction):  Basics.            (line  1182)
* lines, right-justifying (introduction): Basics.           (line  1182)
* list of special characters (groff_char(7) man page): Using Symbols.
                                                            (line  7675)
* listing page location traps (ptr):     Debugging.         (line 12162)
* lists:                                 Paragraphs.        (line  1228)
* ll request, using + and - with:        Numeric Expressions.
                                                            (line  4463)
* localization:                          Manipulating Hyphenation.
                                                            (line  6215)
* localization [ms]:                     ms language and localization.
                                                            (line  3003)
* locating macro files:                  Macro Directories. (line   900)
* locating macro packages:               Macro Directories. (line   900)
* location, vertical, page, marking (mk): Page Motions.     (line  9868)
* location, vertical, page, returning to marked (rt): Page Motions.
                                                            (line  9868)
* logical "and" operator:                Numeric Expressions.
                                                            (line  4422)
* logical "or" operator:                 Numeric Expressions.
                                                            (line  4422)
* logical complementation operator:      Numeric Expressions.
                                                            (line  4426)
* logical conjunction operator:          Numeric Expressions.
                                                            (line  4422)
* logical disjunction operator:          Numeric Expressions.
                                                            (line  4422)
* logical not, limitation in expression: Numeric Expressions.
                                                            (line  4426)
* logical operators:                     Numeric Expressions.
                                                            (line  4422)
* long names:                            Compatibility Mode.
                                                            (line 12404)
* loops and conditionals:                Conditionals and Loops.
                                                            (line  9020)
* lowercasing a string (stringdown):     Strings.           (line  8945)
* ls request, alternative to (pvs):      Changing the Vertical Spacing.
                                                            (line  8584)
* lt request, using + and - with:        Numeric Expressions.
                                                            (line  4463)
* m scaling unit:                        Measurements.      (line  4277)
* M scaling unit:                        Measurements.      (line  4287)
* machine units:                         Page Geometry.     (line  4169)
* macro:                                 Requests and Macros.
                                                            (line  3840)
* macro arguments:                       Calling Macros.    (line  4851)
* macro arguments, and compatibility mode: Gtroff Internals.
                                                            (line 12054)
* macro arguments, and tabs:             Invoking Requests. (line  4789)
* macro directories:                     Macro Directories. (line   898)
* macro file search path:                Macro Directories. (line   900)
* macro name register (\$0):             Parameters.        (line  9657)
* macro names, starting with [ or ], and refer: Identifiers.
                                                            (line  4623)
* macro package:                         Macro Packages.    (line  3963)
* macro package search path:             Macro Directories. (line   900)
* macro package usage, basics of:        Basics.            (line  1058)
* macro package, auxiliary:              Major Macro Packages.
                                                            (line  1408)
* macro package, full-service:           Major Macro Packages.
                                                            (line  1397)
* macro package, introduction:           Macro Package Intro.
                                                            (line   316)
* macro package, major:                  Major Macro Packages.
                                                            (line  1394)
* macro package, minor:                  Major Macro Packages.
                                                            (line  1408)
* macro package, structuring the source of: Invoking Requests.
                                                            (line  4803)
* macro, appending to (am):              Writing Macros.    (line  9533)
* macro, creating alias for (als):       Strings.           (line  8972)
* macro, end-of-input (em):              End-of-input Traps.
                                                            (line 10807)
* macro, parameters (\$):                Parameters.        (line  9616)
* macro, removing (rm):                  Strings.           (line  8967)
* macro, removing alias for (rm):        Strings.           (line  9007)
* macro, renaming (rn):                  Strings.           (line  8964)
* macros, recursive:                     while.             (line  9359)
* macros, searching:                     Macro Directories. (line   898)
* macros, shared name space with strings and diversions: Identifiers.
                                                            (line  4668)
* macros, tutorial for users:            Tutorial for Macro Users.
                                                            (line  1046)
* macros, writing:                       Writing Macros.    (line  9410)
* magnification of a font (fzoom):       Selecting Fonts.   (line  7421)
* major macro package:                   Major Macro Packages.
                                                            (line  1394)
* major version number register (.x):    Built-in Registers.
                                                            (line  5559)
* man macros, custom headers and footers: Optional man extensions.
                                                            (line  1433)
* man macros, Ultrix-specific:           Optional man extensions.
                                                            (line  1451)
* man pages:                             man.               (line  1417)
* manipulating filling and adjustment:   Manipulating Filling and Adjustment.
                                                            (line  5624)
* manipulating hyphenation:              Manipulating Hyphenation.
                                                            (line  5941)
* manipulating spacing:                  Manipulating Spacing.
                                                            (line  6344)
* manipulating type size and vertical spacing: Manipulating Type Size and Vertical Spacing.
                                                            (line  8438)
* manual hyphenation:                    Manipulating Hyphenation.
                                                            (line  5948)
* manual pages:                          man.               (line  1417)
* margin for hyphenation (hym):          Manipulating Hyphenation.
                                                            (line  6290)
* margin glyph (mc):                     Miscellaneous.     (line 11897)
* margin, bottom:                        Page Location Traps.
                                                            (line 10440)
* margin, left (po):                     Line Layout.       (line  6870)
* margin, right:                         Line Layout.       (line  6881)
* margin, top:                           Page Location Traps.
                                                            (line 10440)
* mark, high-water, register (.h):       Diversions.        (line 11007)
* marker, footnote [ms]:                 ms Footnotes.      (line  2920)
* marking vertical page location (mk):   Page Motions.      (line  9868)
* maximum operator:                      Numeric Expressions.
                                                            (line  4407)
* maximum value representable with Roman numerals: Assigning Register Formats.
                                                            (line  5465)
* mdoc macros:                           mdoc.              (line  1552)
* me macro package:                      me.                (line  1559)
* measurement units:                     Measurements.      (line  4232)
* measurements:                          Measurements.      (line  4232)
* measurements, specifying safely:       Default Units.     (line  4341)
* metrics, font:                         Using Fonts.       (line  7295)
* minimal inter-word spacing:            Manipulating Filling and Adjustment.
                                                            (line  5891)
* minimum operator:                      Numeric Expressions.
                                                            (line  4407)
* minimum value representable with Roman numerals: Assigning Register Formats.
                                                            (line  5465)
* minor macro package:                   Major Macro Packages.
                                                            (line  1408)
* minor version number register (.y):    Built-in Registers.
                                                            (line  5563)
* minutes, current time (minutes):       Built-in Registers.
                                                            (line  5583)
* mm macro package:                      mm.                (line  1571)
* mode for constant glyph space (cs):    Artificial Fonts.  (line  8241)
* mode, compatibility:                   Compatibility Mode.
                                                            (line 12404)
* mode, compatibility, and parameters:   Gtroff Internals.  (line 12054)
* mode, copy:                            Copy Mode.         (line  9699)
* mode, copy <1>:                        Copy Mode.         (line  9699)
* mode, copy, and cf request:            I/O.               (line 11521)
* mode, copy, and device request:        Postprocessor Access.
                                                            (line 11744)
* mode, copy, and length request:        Strings.           (line  8909)
* mode, copy, and macro parameters:      Parameters.        (line  9616)
* mode, copy, and output request:        Diversions.        (line 11106)
* mode, copy, and trf request:           I/O.               (line 11521)
* mode, copy, and write request:         I/O.               (line 11683)
* mode, copy, and writec request:        I/O.               (line 11683)
* mode, copy, and writem request:        I/O.               (line 11697)
* mode, copy, and \!:                    Diversions.        (line 11074)
* mode, copy, and \?:                    Operators in Conditionals.
                                                            (line  9116)
* mode, copy, and \? <1>:                Diversions.        (line 11074)
* mode, copy, and \a:                    Leaders.           (line  6658)
* mode, copy, and \t:                    Tabs and Fields.   (line  6485)
* mode, copy, and \V:                    I/O.               (line 11721)
* mode, fill (fi), enabling:             Manipulating Filling and Adjustment.
                                                            (line  5685)
* mode, fill, and break warnings:        Warnings.          (line 12262)
* mode, fill, and inter-sentence space:  Manipulating Filling and Adjustment.
                                                            (line  5898)
* mode, fill, and \c:                    Line Continuation. (line  7054)
* mode, fill, disabling:                 Manipulating Filling and Adjustment.
                                                            (line  5692)
* mode, interpretation:                  Copy Mode.         (line  9708)
* mode, line-tabs:                       Tabs and Fields.   (line  6615)
* mode, no-fill:                         Manipulating Filling and Adjustment.
                                                            (line  5692)
* mode, no-fill, and \c:                 Line Continuation. (line  7062)
* mode, no-space (ns):                   Manipulating Spacing.
                                                            (line  6456)
* mode, nroff:                           troff and nroff Modes.
                                                            (line  6813)
* mode, safer:                           Groff Options.     (line   707)
* mode, safer <1>:                       Macro Directories. (line   910)
* mode, safer <2>:                       Built-in Registers.
                                                            (line  5555)
* mode, safer <3>:                       I/O.               (line 11504)
* mode, safer <4>:                       I/O.               (line 11616)
* mode, safer <5>:                       I/O.               (line 11638)
* mode, safer <6>:                       I/O.               (line 11677)
* mode, safer <7>:                       Safer Mode.        (line 12395)
* mode, troff:                           troff and nroff Modes.
                                                            (line  6813)
* mode, unsafe:                          Groff Options.     (line   783)
* mode, unsafe <1>:                      Macro Directories. (line   910)
* mode, unsafe <2>:                      Built-in Registers.
                                                            (line  5555)
* mode, unsafe <3>:                      I/O.               (line 11504)
* mode, unsafe <4>:                      I/O.               (line 11616)
* mode, unsafe <5>:                      I/O.               (line 11638)
* mode, unsafe <6>:                      I/O.               (line 11677)
* modifying requests:                    Control Characters.
                                                            (line  4765)
* modulus:                               Numeric Expressions.
                                                            (line  4359)
* mom macro package:                     mom.               (line  1581)
* month of the year register (mo):       Built-in Registers.
                                                            (line  5595)
* motion operators:                      Numeric Expressions.
                                                            (line  4457)
* motion quanta:                         Motion Quanta.     (line  4292)
* motion quantum, horizontal:            DESC File Format.  (line 13445)
* motion quantum, horizontal, register (.H): Motion Quanta. (line  4299)
* motion quantum, vertical:              DESC File Format.  (line 13553)
* motion, horizontal (\h):               Page Motions.      (line  9982)
* motion, vertical (\v):                 Page Motions.      (line  9938)
* motions, page:                         Page Motions.      (line  9863)
* mounting a font (fp):                  Font Positions.    (line  7552)
* mounting position:                     Using Fonts.       (line  7295)
* mounting position <1>:                 Using Fonts.       (line  7295)
* mounting, font, automatic:             Selecting Fonts.   (line  7365)
* ms macros:                             ms.                (line  1603)
* ms macros, accent marks:               ms Legacy Features.
                                                            (line  3405)
* ms macros, body text:                  ms Body Text.      (line  2231)
* ms macros, creating table of contents: ms TOC.            (line  3139)
* ms macros, displays:                   ms keeps and displays.
                                                            (line  2743)
* ms macros, document control settings:  ms Document Control Settings.
                                                            (line  1794)
* ms macros, document description:       ms Document Description Macros.
                                                            (line  2132)
* ms macros, equations:                  ms Insertions.     (line  2844)
* ms macros, figures:                    ms Insertions.     (line  2844)
* ms macros, footers:                    ms Headers and Footers.
                                                            (line  3052)
* ms macros, footnotes:                  ms Footnotes.      (line  2920)
* ms macros, fractional type sizes in:   Differences from AT&T ms.
                                                            (line  3315)
* ms macros, general structure:          ms Document Structure.
                                                            (line  1747)
* ms macros, groff differences from AT&T: Differences from AT&T ms.
                                                            (line  3276)
* ms macros, headers:                    ms Headers and Footers.
                                                            (line  3052)
* ms macros, headings:                   Headings in ms.    (line  2352)
* ms macros, keeps:                      ms keeps and displays.
                                                            (line  2743)
* ms macros, language:                   ms language and localization.
                                                            (line  3003)
* ms macros, lists:                      Lists in ms.       (line  2582)
* ms macros, localization:               ms language and localization.
                                                            (line  3003)
* ms macros, margins:                    ms Margins.        (line  3109)
* ms macros, multiple columns:           ms Multiple Columns.
                                                            (line  3117)
* ms macros, naming conventions:         ms Naming Conventions.
                                                            (line  3520)
* ms macros, nested lists:               Indented regions in ms.
                                                            (line  2713)
* ms macros, obtaining typographical symbols: Typographical symbols in ms.
                                                            (line  2261)
* ms macros, page layout:                ms Page Layout.    (line  3045)
* ms macros, paragraph handling:         Paragraphs in ms.  (line  2277)
* ms macros, references:                 ms Insertions.     (line  2844)
* ms macros, special characters:         ms Legacy Features.
                                                            (line  3405)
* ms macros, strings:                    ms Legacy Features.
                                                            (line  3405)
* ms macros, tables:                     ms Insertions.     (line  2844)
* ms macros, text settings:              Text settings in ms.
                                                            (line  2239)
* multi-file documents:                  Debugging.         (line 12103)
* multi-line strings:                    Strings.           (line  8853)
* multi-page table example [ms]:         ms Insertions.     (line  2886)
* multiple columns [ms]:                 ms Multiple Columns.
                                                            (line  3117)
* multiplication:                        Numeric Expressions.
                                                            (line  4359)
* n scaling unit:                        Measurements.      (line  4281)
* name space, common, of macros, diversions, and strings: Identifiers.
                                                            (line  4668)
* name, background color, register (.M): Colors.            (line  8765)
* name, fill color, register (.M):       Colors.            (line  8765)
* name, stroke color, register (.m):     Colors.            (line  8744)
* named character (\C):                  Using Symbols.     (line  7793)
* names, long:                           Compatibility Mode.
                                                            (line 12404)
* naming conventions, ms macros:         ms Naming Conventions.
                                                            (line  3520)
* ne request, and the .trunc register:   Page Location Traps.
                                                            (line 10587)
* ne request, comparison with sv:        Page Control.      (line  7230)
* negating register values:              Setting Registers. (line  5289)
* negation:                              Numeric Expressions.
                                                            (line  4365)
* nested assignments:                    Interpolating Registers.
                                                            (line  5340)
* nested diversions:                     Diversions.        (line 11000)
* nested lists [ms]:                     Indented regions in ms.
                                                            (line  2713)
* nesting level, suppression, register:  Suppressing Output.
                                                            (line 11463)
* new page (bp):                         Page Control.      (line  7178)
* newline character, and translations:   Character Translations.
                                                            (line  6751)
* newline character, in strings, escaping: Strings.         (line  8853)
* newline, as delimiter:                 Delimiters.        (line  5076)
* newline, final, stripping in diversions: Punning Names.   (line 11246)
* next file, processing (nx):            I/O.               (line 11553)
* next free font position register (.fp): Font Positions.   (line  7589)
* next page number register (.pn):       Page Layout.       (line  7112)
* next page number, configuring (pn):    Page Layout.       (line  7107)
* nf request, causing implicit break:    Manipulating Filling and Adjustment.
                                                            (line  5624)
* nl register, and .d:                   Diversions.        (line 11000)
* nl register, difference from .h:       Diversions.        (line 11020)
* nm request, using + and - with:        Numeric Expressions.
                                                            (line  4463)
* no-break control character ('):        Requests and Macros.
                                                            (line  3813)
* no-break control character, changing (c2): Control Characters.
                                                            (line  4738)
* no-fill mode:                          Manipulating Filling and Adjustment.
                                                            (line  5692)
* no-fill mode, and \c:                  Line Continuation. (line  7062)
* no-space mode (ns):                    Manipulating Spacing.
                                                            (line  6456)
* node, output:                          Gtroff Internals.  (line 11970)
* non-printing break point (\:):         Manipulating Hyphenation.
                                                            (line  6008)
* nr request, and warnings:              Warnings.          (line 12313)
* nr request, using + and - with:        Numeric Expressions.
                                                            (line  4463)
* nroff mode:                            troff and nroff Modes.
                                                            (line  6813)
* number formats, assigning to register (af): Assigning Register Formats.
                                                            (line  5413)
* number of registers register (.R):     Built-in Registers.
                                                            (line  5547)
* number, input line, setting (lf):      Debugging.         (line 12103)
* number, page, next, configuring (pn):  Page Layout.       (line  7107)
* numbered glyph (\N):                   Character Translations.
                                                            (line  6741)
* numbered glyph (\N) <1>:               Using Symbols.     (line  7810)
* numbered list, example markup [ms]:    Lists in ms.       (line  2609)
* numbers, line, printing (nm):          Miscellaneous.     (line 11798)
* numeral-width space (\0):              Page Motions.      (line 10014)
* numerals, as delimiters:               Delimiters.        (line  5090)
* numerals, Roman:                       Assigning Register Formats.
                                                            (line  5433)
* numeric expression, valid:             Numeric Expressions.
                                                            (line  4522)
* numeric expressions:                   Numeric Expressions.
                                                            (line  4348)
* object creation:                       Writing Macros.    (line  9558)
* offset, page:                          Page Geometry.     (line  4193)
* offset, page (po):                     Line Layout.       (line  6870)
* open request, and safer mode:          Groff Options.     (line   707)
* opena request, and safer mode:         Groff Options.     (line   707)
* opening brace escape sequence (\}):    Conditional Blocks.
                                                            (line  9250)
* opening file (open):                   I/O.               (line 11671)
* operator, scaling:                     Numeric Expressions.
                                                            (line  4390)
* operators, arithmetic:                 Numeric Expressions.
                                                            (line  4359)
* operators, as delimiters:              Delimiters.        (line  5092)
* operators, comparison:                 Numeric Expressions.
                                                            (line  4416)
* operators, extremum (>?, <?):          Numeric Expressions.
                                                            (line  4407)
* operators, logical:                    Numeric Expressions.
                                                            (line  4422)
* operators, motion:                     Numeric Expressions.
                                                            (line  4457)
* operators, unary arithmetic:           Numeric Expressions.
                                                            (line  4365)
* optical size of a font:                Selecting Fonts.   (line  7421)
* options:                               Groff Options.     (line   478)
* order of evaluation in expressions:    Numeric Expressions.
                                                            (line  4446)
* ordinary character:                    Identifiers.       (line  4577)
* orientation, landscape:                Paper Format.      (line   972)
* orphan:                                Page Control.      (line  7222)
* orphan lines, preventing with ne:      Page Control.      (line  7195)
* os request, and no-space mode:         Page Control.      (line  7233)
* outlined circle, drawing (\D'c ...'):  Drawing Geometric Objects.
                                                            (line 10230)
* outlined ellipse, drawing (\D'e ...'): Drawing Geometric Objects.
                                                            (line 10237)
* outlined polygon, drawing (\D'p ...'): Drawing Geometric Objects.
                                                            (line 10269)
* output and input requests:             I/O.               (line 11469)
* output comparison operator:            Operators in Conditionals.
                                                            (line  9080)
* output device name string (.T):        Groff Options.     (line   772)
* output device name string (.T) <1>:    Strings.           (line  8787)
* output device name string (.T), in other implementations: Other Differences.
                                                            (line 12564)
* output device usage register (.T):     Groff Options.     (line   772)
* output device usage register (.T), incompatibility with AT&T troff: Other Differences.
                                                            (line 12574)
* output devices:                        Output Device Intro.
                                                            (line   359)
* output encoding, ASCII:                Groff Options.     (line   743)
* output encoding, code page 1047:       Groff Options.     (line   755)
* output encoding, EBCDIC:               Groff Options.     (line   755)
* output encoding, ISO 646:              Groff Options.     (line   743)
* output encoding, Latin-1 (ISO 8859-1): Groff Options.     (line   747)
* output encoding, UTF-8:                Groff Options.     (line   751)
* output glyphs, and input characters, compatibility with AT&T troff: Other Differences.
                                                            (line 12609)
* output line break:                     Breaking.          (line  3725)
* output line number register (ln):      Miscellaneous.     (line 11820)
* output line properties:                Manipulating Filling and Adjustment.
                                                            (line  5644)
* output line, continuation (\c):        Line Continuation. (line  7048)
* output line, horizontal position, register (.k): Page Motions.
                                                            (line 10093)
* output node:                           Gtroff Internals.  (line 11970)
* output request, and copy mode:         Diversions.        (line 11106)
* output request, and \!:                Diversions.        (line 11106)
* output, filling, disablement of (nf):  Manipulating Filling and Adjustment.
                                                            (line  5692)
* output, filling, enablement of (fi):   Manipulating Filling and Adjustment.
                                                            (line  5685)
* output, flush (fl):                    Debugging.         (line 12167)
* output, gtroff:                        gtroff Output.     (line 12680)
* output, intermediate:                  gtroff Output.     (line 12690)
* output, suppressing (\O):              Suppressing Output.
                                                            (line 11410)
* output, transparent (cf, trf):         I/O.               (line 11521)
* output, transparent (\!, \?):          Diversions.        (line 11066)
* output, transparent, incompatibilities with AT&T troff: Other Differences.
                                                            (line 12636)
* output, troff:                         gtroff Output.     (line 12690)
* overlapping characters:                Using Symbols.     (line  7882)
* overstriking glyphs (\o):              Page Motions.      (line 10097)
* p scaling unit:                        Measurements.      (line  4255)
* P scaling unit:                        Measurements.      (line  4259)
* package, macro:                        Macro Packages.    (line  3963)
* package, macro, auxiliary:             Major Macro Packages.
                                                            (line  1408)
* package, macro, full-service:          Major Macro Packages.
                                                            (line  1397)
* package, macro, introduction:          Macro Package Intro.
                                                            (line   316)
* package, macro, major:                 Major Macro Packages.
                                                            (line  1394)
* package, macro, minor:                 Major Macro Packages.
                                                            (line  1408)
* package, macro, search path:           Macro Directories. (line   900)
* package, package, structuring the source of: Invoking Requests.
                                                            (line  4803)
* padding character, for fields (fc):    Fields.            (line  6692)
* page:                                  Page Geometry.     (line  4175)
* page break:                            Page Geometry.     (line  4208)
* page break <1>:                        Page Control.      (line  7170)
* page break <2>:                        The Implicit Page Trap.
                                                            (line 10638)
* page break (introduction):             Basics.            (line  1185)
* page break, conditional (ne):          Page Control.      (line  7195)
* page break, final:                     End-of-input Traps.
                                                            (line 10834)
* page break, prevented by vpt:          Vertical Position Traps.
                                                            (line 10414)
* page control:                          Page Control.      (line  7170)
* page ejection:                         Page Geometry.     (line  4208)
* page ejection <1>:                     Page Control.      (line  7170)
* page ejection <2>:                     The Implicit Page Trap.
                                                            (line 10638)
* page ejection status register (.pe):   Page Location Traps.
                                                            (line 10598)
* page ejection, of final page:          End-of-input Traps.
                                                            (line 10834)
* page ejection, prevented by vpt:       Vertical Position Traps.
                                                            (line 10414)
* page footers:                          Page Location Traps.
                                                            (line 10440)
* page headers:                          Page Location Traps.
                                                            (line 10440)
* page layout:                           Page Layout.       (line  7087)
* page layout [ms]:                      ms Page Layout.    (line  3045)
* page length register (.p):             Page Layout.       (line  7101)
* page length, configuring (pl):         Page Layout.       (line  7093)
* page location traps:                   Page Location Traps.
                                                            (line 10420)
* page location traps, debugging:        Page Location Traps.
                                                            (line 10476)
* page location, vertical, marking (mk): Page Motions.      (line  9868)
* page location, vertical, returning to marked (rt): Page Motions.
                                                            (line  9868)
* page motions:                          Page Motions.      (line  9863)
* page number character (%):             Page Layout.       (line  7120)
* page number character, changing (pc):  Page Layout.       (line  7149)
* page number register (%):              Page Control.      (line  7188)
* page number, configuring next (pn):    Page Layout.       (line  7107)
* page number, next, register (.pn):     Page Layout.       (line  7112)
* page offset:                           Page Geometry.     (line  4193)
* page offset (po):                      Line Layout.       (line  6870)
* page orientation, landscape:           Paper Format.      (line   972)
* page, geometry of:                     Page Geometry.     (line  4162)
* page, new (bp):                        Page Control.      (line  7178)
* paper format:                          Paper Format.      (line   972)
* paper size:                            Paper Format.      (line   972)
* paragraphs:                            Paragraphs.        (line  1220)
* parameter count register (.$):         Parameters.        (line  9594)
* parameters:                            Parameters.        (line  9586)
* parameters, and compatibility mode:    Gtroff Internals.  (line 12054)
* parameters, macro (\$):                Parameters.        (line  9616)
* parentheses:                           Numeric Expressions.
                                                            (line  4446)
* partially collected line:              Manipulating Filling and Adjustment.
                                                            (line  5644)
* path, for font files:                  Font Directories.  (line   942)
* path, for tmac files:                  Macro Directories. (line   900)
* pattern files, for hyphenation:        Manipulating Hyphenation.
                                                            (line  6116)
* patterns for hyphenation (hpf):        Manipulating Hyphenation.
                                                            (line  6170)
* pending output line:                   Manipulating Filling and Adjustment.
                                                            (line  5644)
* pi request, and groff:                 I/O.               (line 11629)
* pi request, and safer mode:            Groff Options.     (line   707)
* pi request, disabled by default:       Safer Mode.        (line 12395)
* pica scaling unit (P):                 Measurements.      (line  4259)
* PID of GNU troff register ($$):        Built-in Registers.
                                                            (line  5571)
* pile, glyph (\b):                      Drawing Geometric Objects.
                                                            (line 10308)
* pl request, using + and - with:        Numeric Expressions.
                                                            (line  4463)
* plain text approximation output register (.A): Groff Options.
                                                            (line   521)
* plain text approximation output register (.A) <1>: Built-in Registers.
                                                            (line  5527)
* planting a trap:                       Traps.             (line 10393)
* platform-specific directory:           Macro Directories. (line   915)
* pm request, incompatibilities with AT&T troff: Other Differences.
                                                            (line 12600)
* pn request, using + and - with:        Numeric Expressions.
                                                            (line  4463)
* PNG image generation from PostScript:  DESC File Format.  (line 13449)
* po request, using + and - with:        Numeric Expressions.
                                                            (line  4463)
* point scaling unit (p):                Measurements.      (line  4255)
* point size registers (.s, .ps):        Changing the Type Size.
                                                            (line  8480)
* point size registers, last-requested (.psr, .sr): Using Fractional Type Sizes.
                                                            (line  8638)
* point sizes, changing (ps, \s):        Changing the Type Size.
                                                            (line  8466)
* point sizes, fractional:               Using Fractional Type Sizes.
                                                            (line  8597)
* point sizes, fractional <1>:           Other Differences. (line 12587)
* polygon, filled, drawing (\D'P ...'):  Drawing Geometric Objects.
                                                            (line 10275)
* polygon, outlined, drawing (\D'p ...'): Drawing Geometric Objects.
                                                            (line 10269)
* polygon, solid, drawing (\D'P ...'):   Drawing Geometric Objects.
                                                            (line 10275)
* polygon, stroked, drawing (\D'p ...'): Drawing Geometric Objects.
                                                            (line 10269)
* position of lowest text line (.h):     Diversions.        (line 11007)
* position, absolute (sic) operator (|): Numeric Expressions.
                                                            (line  4471)
* position, drawing:                     Page Geometry.     (line  4181)
* position, horizontal input line, saving (\k): Page Motions.
                                                            (line 10084)
* position, horizontal, in input line, register (hp): Page Motions.
                                                            (line 10090)
* position, horizontal, in output line, register (.k): Page Motions.
                                                            (line 10093)
* position, mounting:                    Using Fonts.       (line  7295)
* position, vertical, in diversion, register (.d): Diversions.
                                                            (line 11000)
* positions, font:                       Font Positions.    (line  7540)
* post-vertical line spacing:            Changing the Vertical Spacing.
                                                            (line  8572)
* post-vertical line spacing register (.pvs): Changing the Vertical Spacing.
                                                            (line  8584)
* post-vertical line spacing, changing (pvs): Changing the Vertical Spacing.
                                                            (line  8584)
* postprocessor access:                  Postprocessor Access.
                                                            (line 11729)
* postprocessors:                        Output Device Intro.
                                                            (line   359)
* PostScript, bounding box:              Miscellaneous.     (line 11949)
* PostScript, PNG image generation:      DESC File Format.  (line 13449)
* prefix, for commands:                  Environment.       (line   835)
* preprocessors:                         Preprocessor Intro.
                                                            (line   328)
* previous font, selecting (ft):         Selecting Fonts.   (line  7352)
* previous font, selecting (\f[], \fP):  Selecting Fonts.   (line  7378)
* previous line length (.n):             Environments.      (line 11403)
* print current page register (.P):      Groff Options.     (line   670)
* printing backslash (\\, \e, \E, \[rs]): Other Differences.
                                                            (line 12636)
* printing line numbers (nm):            Miscellaneous.     (line 11798)
* printing to stderr (tm, tm1, tmc):     Debugging.         (line 12117)
* printing, zero-width (\z, \Z):         Page Motions.      (line 10102)
* printing, zero-width (\z, \Z) <1>:     Page Motions.      (line 10107)
* process ID of GNU troff register ($$): Built-in Registers.
                                                            (line  5571)
* processing next file (nx):             I/O.               (line 11553)
* productive input line:                 Manipulating Filling and Adjustment.
                                                            (line  5797)
* properties of characters (cflags):     Using Symbols.     (line  7846)
* properties of glyphs (cflags):         Using Symbols.     (line  7846)
* properties of output lines:            Manipulating Filling and Adjustment.
                                                            (line  5644)
* ps request, and constant glyph space mode: Artificial Fonts.
                                                            (line  8241)
* ps request, incompatibilities with AT&T troff: Other Differences.
                                                            (line 12587)
* ps request, using + and - with:        Numeric Expressions.
                                                            (line  4463)
* ps request, with fractional type sizes: Using Fractional Type Sizes.
                                                            (line  8604)
* pso request, and safer mode:           Groff Options.     (line   707)
* pvs request, using + and - with:       Numeric Expressions.
                                                            (line  4463)
* quanta, motion:                        Motion Quanta.     (line  4292)
* quantum, horizontal motion:            DESC File Format.  (line 13445)
* quantum, vertical motion:              DESC File Format.  (line 13553)
* radicalex glyph, and cflags:           Using Symbols.     (line  7882)
* ragged-left text:                      Manipulating Filling and Adjustment.
                                                            (line  5723)
* ragged-right text:                     Manipulating Filling and Adjustment.
                                                            (line  5720)
* rc request, and glyph definitions:     Using Symbols.     (line  7948)
* read-only register removal, incompatibility with AT&T troff: Other Differences.
                                                            (line 12571)
* read-only register, changing format:   Assigning Register Formats.
                                                            (line  5472)
* reading from standard input (rd):      I/O.               (line 11558)
* recursive macros:                      while.             (line  9359)
* refer, and macro names starting with [ or ]: Identifiers. (line  4623)
* reference, gtroff:                     GNU troff Reference.
                                                            (line  3549)
* references [ms]:                       ms Insertions.     (line  2844)
* register format, in expressions:       Assigning Register Formats.
                                                            (line  5484)
* register, assigning number format to (af): Assigning Register Formats.
                                                            (line  5413)
* register, built-in, removing:          Built-in Registers.
                                                            (line  5513)
* register, creating alias for (aln):    Setting Registers. (line  5323)
* register, format (\g):                 Assigning Register Formats.
                                                            (line  5479)
* register, read-only, removal, incompatibility with AT&T troff: Other Differences.
                                                            (line 12571)
* register, removing (rr):               Setting Registers. (line  5312)
* register, removing alias for (rr):     Setting Registers. (line  5329)
* register, renaming (rnn):              Setting Registers. (line  5318)
* registers:                             Registers.         (line  5210)
* registers, built-in:                   Built-in Registers.
                                                            (line  5508)
* registers, dumping (pnr):              Debugging.         (line 12158)
* registers, interpolating (\n):         Interpolating Registers.
                                                            (line  5335)
* registers, number of, register (.R):   Built-in Registers.
                                                            (line  5547)
* registers, setting (nr, \R):           Setting Registers. (line  5219)
* removal of read-only registers, incompatibility with AT&T troff: Other Differences.
                                                            (line 12571)
* removing a built-in register:          Built-in Registers.
                                                            (line  5513)
* removing a register (rr):              Setting Registers. (line  5312)
* removing alias for register (rr):      Setting Registers. (line  5329)
* removing alias, for diversion (rm):    Strings.           (line  9007)
* removing alias, for macro (rm):        Strings.           (line  9007)
* removing alias, for string (rm):       Strings.           (line  9007)
* removing diversion (rm):               Strings.           (line  8967)
* removing glyph definition (rchar, rfschar): Using Symbols.
                                                            (line  8005)
* removing macro (rm):                   Strings.           (line  8967)
* removing request (rm):                 Strings.           (line  8967)
* removing string (rm):                  Strings.           (line  8967)
* renaming a register (rnn):             Setting Registers. (line  5318)
* renaming diversion (rn):               Strings.           (line  8964)
* renaming macro (rn):                   Strings.           (line  8964)
* renaming request (rn):                 Strings.           (line  8964)
* renaming string (rn):                  Strings.           (line  8964)
* renditions, graphic:                   Using Fonts.       (line  7323)
* request:                               Requests and Macros.
                                                            (line  3813)
* request <1>:                           Formatter Instructions.
                                                            (line  4708)
* request arguments:                     Invoking Requests. (line  4789)
* request arguments, and compatibility mode: Gtroff Internals.
                                                            (line 12054)
* request arguments, and tabs:           Invoking Requests. (line  4789)
* request, removing (rm):                Strings.           (line  8967)
* request, renaming (rn):                Strings.           (line  8964)
* request, undefined:                    Comments.          (line  5150)
* requests for drawing:                  Drawing Geometric Objects.
                                                            (line 10130)
* requests for input and output:         I/O.               (line 11469)
* requests, intercepting:                Control Characters.
                                                            (line  4765)
* requests, invoking:                    Invoking Requests. (line  4781)
* requests, modifying:                   Control Characters.
                                                            (line  4765)
* resolution, device:                    Page Geometry.     (line  4169)
* resolution, device <1>:                DESC File Format.  (line 13506)
* resolution, device, obtaining in the formatter: Measurements.
                                                            (line  4240)
* resolution, horizontal:                DESC File Format.  (line 13445)
* resolution, horizontal, register (.H): Motion Quanta.     (line  4299)
* resolution, vertical:                  DESC File Format.  (line 13553)
* returning to marked vertical page location (rt): Page Motions.
                                                            (line  9868)
* revision number register (.Y):         Built-in Registers.
                                                            (line  5567)
* right margin:                          Line Layout.       (line  6881)
* right-aligning lines (introduction):   Basics.            (line  1182)
* right-justifying (rj):                 Manipulating Filling and Adjustment.
                                                            (line  5866)
* right-justifying lines (introduction): Basics.            (line  1182)
* rivers:                                Other Differences. (line 12552)
* rj request, causing implicit break:    Manipulating Filling and Adjustment.
                                                            (line  5624)
* rn glyph, and cflags:                  Using Symbols.     (line  7882)
* roman glyph, correction after italic glyph (\/): Italic Corrections.
                                                            (line  8328)
* roman glyph, correction before italic glyph (\,): Italic Corrections.
                                                            (line  8338)
* Roman numerals:                        Assigning Register Formats.
                                                            (line  5433)
* Roman numerals, extrema (maximum and minimum): Assigning Register Formats.
                                                            (line  5465)
* rq glyph, at end of sentence:          Sentences.         (line  3663)
* rq glyph, at end of sentence <1>:      Using Symbols.     (line  7892)
* rt request, using + and - with:        Numeric Expressions.
                                                            (line  4463)
* ru glyph, and cflags:                  Using Symbols.     (line  7882)
* running system commands:               I/O.               (line 11633)
* s scaling unit:                        Using Fractional Type Sizes.
                                                            (line  8604)
* safer mode:                            Groff Options.     (line   707)
* safer mode <1>:                        Macro Directories. (line   910)
* safer mode <2>:                        Built-in Registers.
                                                            (line  5555)
* safer mode <3>:                        I/O.               (line 11504)
* safer mode <4>:                        I/O.               (line 11616)
* safer mode <5>:                        I/O.               (line 11638)
* safer mode <6>:                        I/O.               (line 11677)
* safer mode <7>:                        Safer Mode.        (line 12395)
* saving horizontal input line position (\k): Page Motions. (line 10084)
* scaling indicator:                     Measurements.      (line  4232)
* scaling operator:                      Numeric Expressions.
                                                            (line  4390)
* scaling unit c:                        Measurements.      (line  4252)
* scaling unit f:                        Colors.            (line  8712)
* scaling unit i:                        Measurements.      (line  4249)
* scaling unit m:                        Measurements.      (line  4277)
* scaling unit M:                        Measurements.      (line  4287)
* scaling unit n:                        Measurements.      (line  4281)
* scaling unit p:                        Measurements.      (line  4255)
* scaling unit P:                        Measurements.      (line  4259)
* scaling unit s:                        Using Fractional Type Sizes.
                                                            (line  8604)
* scaling unit u:                        Measurements.      (line  4245)
* scaling unit v:                        Measurements.      (line  4284)
* scaling unit z:                        Using Fractional Type Sizes.
                                                            (line  8604)
* searching fonts:                       Font Directories.  (line   934)
* searching macros:                      Macro Directories. (line   898)
* seconds, current time (seconds):       Built-in Registers.
                                                            (line  5580)
* selecting the previous font (ft):      Selecting Fonts.   (line  7352)
* sentence space:                        Sentences.         (line  3619)
* sentence space size register (.sss):   Manipulating Filling and Adjustment.
                                                            (line  5887)
* sentences:                             Sentences.         (line  3607)
* sequence, escape:                      Formatter Instructions.
                                                            (line  4714)
* setting diversion trap (dt):           Diversion Traps.   (line 10668)
* setting end-of-input trap (em):        End-of-input Traps.
                                                            (line 10807)
* setting input line number (lf):        Debugging.         (line 12103)
* setting input line trap (it, itc):     Input Line Traps.  (line 10680)
* setting registers (nr, \R):            Setting Registers. (line  5219)
* setting the page length (pl):          Page Layout.       (line  7093)
* setting up an abstract font style (sty): Font Families.   (line  7505)
* shc request, and translations:         Character Translations.
                                                            (line  6755)
* site-local directory:                  Macro Directories. (line   915)
* site-local directory <1>:              Font Directories.  (line   957)
* size of sentence space register (.sss): Manipulating Filling and Adjustment.
                                                            (line  5887)
* size of word space register (.ss):     Manipulating Filling and Adjustment.
                                                            (line  5887)
* size, optical, of a font:              Selecting Fonts.   (line  7421)
* size, paper:                           Paper Format.      (line   972)
* size, size:                            Manipulating Type Size and Vertical Spacing.
                                                            (line  8438)
* sizes, fractional:                     Other Differences. (line 12587)
* sizes, fractional type:                Using Fractional Type Sizes.
                                                            (line  8597)
* skew, of last glyph (.csk):            Environments.      (line 11388)
* slant, font, changing (\S):            Artificial Fonts.  (line  8161)
* soft hyphen character, setting (shc):  Manipulating Hyphenation.
                                                            (line  6032)
* soft hyphen glyph (hy):                Manipulating Hyphenation.
                                                            (line  6032)
* solid circle, drawing (\D'C ...'):     Drawing Geometric Objects.
                                                            (line 10234)
* solid ellipse, drawing (\D'E ...'):    Drawing Geometric Objects.
                                                            (line 10241)
* solid polygon, drawing (\D'P ...'):    Drawing Geometric Objects.
                                                            (line 10275)
* SOURCE_DATE_EPOCH, environment variable: Environment.     (line   882)
* sp request, and no-space mode:         Manipulating Spacing.
                                                            (line  6456)
* sp request, causing implicit break:    Manipulating Filling and Adjustment.
                                                            (line  5624)
* space between sentences:               Sentences.         (line  3619)
* space between sentences register (.sss): Manipulating Filling and Adjustment.
                                                            (line  5887)
* space between words register (.ss):    Manipulating Filling and Adjustment.
                                                            (line  5887)
* space character, as delimiter:         Delimiters.        (line  5094)
* space characters, in expressions:      Numeric Expressions.
                                                            (line  4538)
* space, between sentences:              Manipulating Filling and Adjustment.
                                                            (line  5893)
* space, between words:                  Manipulating Filling and Adjustment.
                                                            (line  5891)
* space, discardable, horizontal:        Manipulating Filling and Adjustment.
                                                            (line  5914)
* space, hair (\^):                      Page Motions.      (line 10008)
* space, horizontal (\h):                Page Motions.      (line  9982)
* space, horizontal, unformatting:       Punning Names.     (line 11246)
* space, thin (\|):                      Page Motions.      (line 10003)
* space, unbreakable (\~):               Manipulating Filling and Adjustment.
                                                            (line  5671)
* space, unbreakable and unadjustable (\<SP>): Page Motions.
                                                            (line  9998)
* space, vertical, unit (v):             Measurements.      (line  4284)
* space, width of a digit (numeral) (\0): Page Motions.     (line 10014)
* spaces with ds:                        Strings.           (line  8844)
* spaces, in a macro argument:           Calling Macros.    (line  4856)
* spaces, leading and trailing:          Breaking.          (line  3757)
* spacing (introduction):                Basics.            (line  1150)
* spacing, manipulating:                 Manipulating Spacing.
                                                            (line  6344)
* spacing, vertical:                     Page Geometry.     (line  4202)
* spacing, vertical <1>:                 Manipulating Type Size and Vertical Spacing.
                                                            (line  8438)
* spacing, vertical (introduction):      Basics.            (line  1136)
* special characters:                    Sentences.         (line  3663)
* special characters <1>:                Character Translations.
                                                            (line  6741)
* special characters [ms]:               ms Legacy Features.
                                                            (line  3405)
* special characters, list of (groff_char(7) man page): Using Symbols.
                                                            (line  7675)
* special font:                          Using Fonts.       (line  7280)
* special fonts:                         Using Symbols.     (line  7612)
* special fonts <1>:                     Special Fonts.     (line  8092)
* special fonts <2>:                     Font Description File Format.
                                                            (line 13618)
* special fonts, emboldening:            Artificial Fonts.  (line  8230)
* special request, and font translations: Selecting Fonts.  (line  7406)
* special request, and glyph search order: Using Symbols.   (line  7612)
* spline, drawing (\D'~ ...'):           Drawing Geometric Objects.
                                                            (line 10221)
* springing a trap:                      Traps.             (line 10394)
* sqrtex glyph, and cflags:              Using Symbols.     (line  7882)
* ss request, incompatibilities with AT&T troff: Other Differences.
                                                            (line 12604)
* stack:                                 Environments.      (line 11273)
* stacking glyphs (\b):                  Drawing Geometric Objects.
                                                            (line 10308)
* standard input, reading from (rd):     I/O.               (line 11558)
* stderr, printing to (tm, tm1, tmc):    Debugging.         (line 12117)
* stops, tab:                            Tabs and Leaders.  (line  3785)
* string arguments:                      Strings.           (line  8802)
* string comparison:                     Operators in Conditionals.
                                                            (line  9108)
* string expansion (\*):                 Strings.           (line  8802)
* string interpolation (\*):             Strings.           (line  8802)
* string, appending (as):                Strings.           (line  8889)
* string, creating alias for (als):      Strings.           (line  8972)
* string, length of (length):            Strings.           (line  8909)
* string, removing (rm):                 Strings.           (line  8967)
* string, removing alias for (rm):       Strings.           (line  9007)
* string, renaming (rn):                 Strings.           (line  8964)
* strings:                               Strings.           (line  8780)
* strings [ms]:                          ms Legacy Features.
                                                            (line  3405)
* strings, multi-line:                   Strings.           (line  8853)
* strings, shared name space with macros and diversions: Identifiers.
                                                            (line  4668)
* stripping final newline in diversions: Punning Names.     (line 11246)
* stroke color:                          Colors.            (line  8678)
* stroke color name register (.m):       Colors.            (line  8744)
* stroked circle, drawing (\D'c ...'):   Drawing Geometric Objects.
                                                            (line 10230)
* stroked ellipse, drawing (\D'e ...'):  Drawing Geometric Objects.
                                                            (line 10237)
* stroked polygon, drawing (\D'p ...'):  Drawing Geometric Objects.
                                                            (line 10269)
* structuring source code of documents or macro packages: Invoking Requests.
                                                            (line  4803)
* sty request, and changing fonts:       Selecting Fonts.   (line  7352)
* sty request, and font translations:    Selecting Fonts.   (line  7406)
* style, font:                           Using Fonts.       (line  7280)
* style, font, abstract:                 Using Fonts.       (line  7302)
* style, font, abstract, setting up (sty): Font Families.   (line  7505)
* styles, font:                          Font Families.     (line  7448)
* substring (substring):                 Strings.           (line  8927)
* subtraction:                           Numeric Expressions.
                                                            (line  4359)
* suppressing output (\O):               Suppressing Output.
                                                            (line 11410)
* suppression nesting level register:    Suppressing Output.
                                                            (line 11463)
* sv request, and no-space mode:         Page Control.      (line  7233)
* switching environments (ev):           Environments.      (line 11309)
* sy request, and safer mode:            Groff Options.     (line   707)
* sy request, disabled by default:       Safer Mode.        (line 12395)
* symbol:                                Using Symbols.     (line  7612)
* symbol table, dumping (pm):            Debugging.         (line 12154)
* symbol, defining (char):               Using Symbols.     (line  7948)
* symbols, using:                        Using Symbols.     (line  7600)
* system commands, running:              I/O.               (line 11633)
* system() return value register (systat): I/O.             (line 11666)
* tab character:                         Tabs and Leaders.  (line  3785)
* tab character encoding:                Tabs and Fields.   (line  6481)
* tab character, and translations:       Character Translations.
                                                            (line  6751)
* tab character, as delimiter:           Delimiters.        (line  5094)
* tab character, non-interpreted (\t):   Tabs and Fields.   (line  6485)
* tab repetition character (tc):         Tabs and Fields.   (line  6602)
* tab stop settings register (.tabs):    Tabs and Fields.   (line  6593)
* tab stops:                             Tabs and Leaders.  (line  3785)
* tab stops, default:                    Tabs and Fields.   (line  6493)
* tab, line-tabs mode:                   Tabs and Fields.   (line  6615)
* table of contents:                     Table of Contents. (line  1323)
* table of contents <1>:                 Leaders.           (line  6671)
* table of contents, creating [ms]:      ms TOC.            (line  3139)
* table, multi-page, example [ms]:       ms Insertions.     (line  2886)
* tables [ms]:                           ms Insertions.     (line  2844)
* tabs, and fields:                      Tabs and Fields.   (line  6481)
* tabs, and macro arguments:             Invoking Requests. (line  4789)
* tabs, and request arguments:           Invoking Requests. (line  4789)
* tabs, before comments:                 Comments.          (line  5145)
* tagged paragraphs:                     Paragraphs.        (line  1228)
* tags, paragraph:                       Paragraphs.        (line  1228)
* terminal, conditional output for:      Operators in Conditionals.
                                                            (line  9053)
* text baseline:                         Page Geometry.     (line  4184)
* text baseline <1>:                     Manipulating Type Size and Vertical Spacing.
                                                            (line  8438)
* text font:                             Using Fonts.       (line  7280)
* text line:                             Requests and Macros.
                                                            (line  3826)
* text line, position of lowest (.h):    Diversions.        (line 11007)
* text, GNU troff processing:            Text.              (line  3556)
* text, justifying:                      Manipulating Filling and Adjustment.
                                                            (line  5624)
* text, justifying (rj):                 Manipulating Filling and Adjustment.
                                                            (line  5866)
* thickness of lines (\D't ...'):        Drawing Geometric Objects.
                                                            (line 10298)
* thin space (\|):                       Page Motions.      (line 10003)
* three-part title (tl):                 Page Layout.       (line  7120)
* ti request, causing implicit break:    Manipulating Filling and Adjustment.
                                                            (line  5624)
* ti request, using + and - with:        Numeric Expressions.
                                                            (line  4463)
* time, current, hours (hours):          Built-in Registers.
                                                            (line  5586)
* time, current, minutes (minutes):      Built-in Registers.
                                                            (line  5583)
* time, current, seconds (seconds):      Built-in Registers.
                                                            (line  5580)
* time, formatting:                      I/O.               (line 11656)
* title length, configuring (lt):        Page Layout.       (line  7137)
* title line length register (.lt):      Page Layout.       (line  7146)
* title line, formatting (tl):           Page Layout.       (line  7120)
* titles:                                Page Layout.       (line  7115)
* tkf request, and font styles:          Font Families.     (line  7505)
* tkf request, and font translations:    Selecting Fonts.   (line  7406)
* tkf request, with fractional type sizes: Using Fractional Type Sizes.
                                                            (line  8604)
* tl request, and mc:                    Miscellaneous.     (line 11901)
* tmac, directory:                       Macro Directories. (line   900)
* tmac, path:                            Macro Directories. (line   900)
* TMPDIR, environment variable:          Environment.       (line   869)
* token, input:                          Gtroff Internals.  (line 11970)
* top margin:                            Page Location Traps.
                                                            (line 10440)
* top-level diversion:                   Diversions.        (line 10930)
* top-level diversion, and bp:           Page Control.      (line  7183)
* top-level diversion, and \!:           Diversions.        (line 11098)
* top-level diversion, and \?:           Diversions.        (line 11103)
* tr request, and glyph definitions:     Using Symbols.     (line  7948)
* tr request, and soft hyphen character: Manipulating Hyphenation.
                                                            (line  6032)
* tr request, incompatibilities with AT&T troff: Other Differences.
                                                            (line 12609)
* track kerning:                         Ligatures and Kerning.
                                                            (line  8300)
* track kerning, activating (tkf):       Ligatures and Kerning.
                                                            (line  8307)
* trailing double quotes in strings:     Strings.           (line  8844)
* trailing spaces in string definitions and appendments: Strings.
                                                            (line  8823)
* trailing spaces on text lines:         Breaking.          (line  3757)
* translations of characters:            Character Translations.
                                                            (line  6722)
* transparent characters:                Using Symbols.     (line  7892)
* transparent dummy character (\)):      Dummy Characters.  (line  8407)
* transparent output (cf, trf):          I/O.               (line 11521)
* transparent output (\!, \?):           Diversions.        (line 11066)
* transparent output, incompatibilities with AT&T troff: Other Differences.
                                                            (line 12636)
* trap:                                  Deferring Output.  (line 10337)
* trap, changing location (ch):          Page Location Traps.
                                                            (line 10528)
* trap, distance to next vertical position, register (.t): Page Location Traps.
                                                            (line 10520)
* trap, diversion, setting (dt):         Diversion Traps.   (line 10668)
* trap, end-of-input, setting (em):      End-of-input Traps.
                                                            (line 10807)
* trap, implicit:                        The Implicit Page Trap.
                                                            (line 10638)
* trap, input line, clearing (it, itc):  Input Line Traps.  (line 10680)
* trap, input line, setting (it, itc):   Input Line Traps.  (line 10680)
* trap, planting:                        Traps.             (line 10393)
* trap, springing:                       Traps.             (line 10394)
* traps:                                 Traps.             (line 10387)
* traps, and diversions:                 Page Location Traps.
                                                            (line 10618)
* traps, blank line:                     Blank Line Traps.  (line 10778)
* traps, diversion:                      Diversion Traps.   (line 10663)
* traps, end-of-input:                   End-of-input Traps.
                                                            (line 10806)
* traps, input line:                     Input Line Traps.  (line 10678)
* traps, input line, and interrupted lines (itc): Input Line Traps.
                                                            (line 10704)
* traps, leading space:                  Leading Space Traps.
                                                            (line 10788)
* traps, page location:                  Page Location Traps.
                                                            (line 10420)
* traps, page location, dumping (ptr):   Debugging.         (line 12162)
* traps, page location, listing (ptr):   Debugging.         (line 12162)
* traps, sprung by bp request (.pe):     Page Location Traps.
                                                            (line 10598)
* traps, vertical position:              Vertical Position Traps.
                                                            (line 10399)
* trf request, and copy mode:            I/O.               (line 11521)
* trf request, and invalid characters:   I/O.               (line 11533)
* trf request, causing implicit break:   Manipulating Filling and Adjustment.
                                                            (line  5624)
* trin request, and asciify:             Diversions.        (line 11120)
* troff mode:                            troff and nroff Modes.
                                                            (line  6813)
* troff output:                          gtroff Output.     (line 12690)
* truncated vertical space register (.trunc): Page Location Traps.
                                                            (line 10587)
* truncating division:                   Numeric Expressions.
                                                            (line  4359)
* TTY, conditional output for:           Operators in Conditionals.
                                                            (line  9053)
* tutorial for macro users:              Tutorial for Macro Users.
                                                            (line  1046)
* type size:                             Manipulating Type Size and Vertical Spacing.
                                                            (line  8438)
* type size registers (.s, .ps):         Changing the Type Size.
                                                            (line  8480)
* type size registers, last-requested (.psr, .sr): Using Fractional Type Sizes.
                                                            (line  8638)
* type sizes, changing (ps, \s):         Changing the Type Size.
                                                            (line  8466)
* type sizes, fractional:                Using Fractional Type Sizes.
                                                            (line  8597)
* type sizes, fractional <1>:            Other Differences. (line 12587)
* typeface:                              Using Fonts.       (line  7280)
* TZ, environment variable:              Environment.       (line   889)
* u scaling unit:                        Measurements.      (line  4245)
* uf request, and font styles:           Font Families.     (line  7505)
* ul glyph, and cflags:                  Using Symbols.     (line  7882)
* ul request, and font translations:     Selecting Fonts.   (line  7406)
* Ultrix-specific man macros:            Optional man extensions.
                                                            (line  1451)
* unadjustable and unbreakable space (\<SP>): Page Motions. (line  9998)
* unary arithmetic operators:            Numeric Expressions.
                                                            (line  4365)
* unbreakable and unadjustable space (\<SP>): Page Motions. (line  9998)
* unbreakable space (\~):                Manipulating Filling and Adjustment.
                                                            (line  5671)
* undefined identifiers:                 Identifiers.       (line  4656)
* undefined request:                     Comments.          (line  5150)
* underline font (uf):                   Artificial Fonts.  (line  8206)
* underlining (ul):                      Artificial Fonts.  (line  8180)
* underlining, continuous (cu):          Artificial Fonts.  (line  8202)
* unformatting diversions (asciify):     Diversions.        (line 11120)
* unformatting horizontal space:         Punning Names.     (line 11246)
* Unicode:                               Identifiers.       (line  4581)
* Unicode <1>:                           Using Symbols.     (line  7810)
* unit, scaling, c:                      Measurements.      (line  4252)
* unit, scaling, f:                      Colors.            (line  8712)
* unit, scaling, i:                      Measurements.      (line  4249)
* unit, scaling, m:                      Measurements.      (line  4277)
* unit, scaling, M:                      Measurements.      (line  4287)
* unit, scaling, n:                      Measurements.      (line  4281)
* unit, scaling, p:                      Measurements.      (line  4255)
* unit, scaling, P:                      Measurements.      (line  4259)
* unit, scaling, s:                      Using Fractional Type Sizes.
                                                            (line  8604)
* unit, scaling, u:                      Measurements.      (line  4245)
* unit, scaling, v:                      Measurements.      (line  4284)
* unit, scaling, z:                      Using Fractional Type Sizes.
                                                            (line  8604)
* units of measurement:                  Measurements.      (line  4232)
* units, basic:                          Page Geometry.     (line  4169)
* units, basic, conversion to:           Measurements.      (line  4239)
* units, default:                        Default Units.     (line  4318)
* units, machine:                        Page Geometry.     (line  4169)
* unnamed glyphs:                        Using Symbols.     (line  7820)
* unnamed glyphs, accessing with \N:     Font Description File Format.
                                                            (line 13639)
* unsafe mode:                           Groff Options.     (line   783)
* unsafe mode <1>:                       Macro Directories. (line   910)
* unsafe mode <2>:                       Built-in Registers.
                                                            (line  5555)
* unsafe mode <3>:                       I/O.               (line 11504)
* unsafe mode <4>:                       I/O.               (line 11616)
* unsafe mode <5>:                       I/O.               (line 11638)
* unsafe mode <6>:                       I/O.               (line 11677)
* unstyled font:                         Using Fonts.       (line  7280)
* up-casing a string (stringup):         Strings.           (line  8945)
* uppercasing a string (stringup):       Strings.           (line  8945)
* upright glyph, correction after oblique glyph (\/): Italic Corrections.
                                                            (line  8328)
* upright glyph, correction before oblique glyph (\,): Italic Corrections.
                                                            (line  8338)
* URLs, breaking (\:):                   Manipulating Hyphenation.
                                                            (line  6008)
* user's macro tutorial:                 Tutorial for Macro Users.
                                                            (line  1046)
* user's tutorial for macros:            Tutorial for Macro Users.
                                                            (line  1046)
* using escape sequences:                Using Escape Sequences.
                                                            (line  4942)
* using symbols:                         Using Symbols.     (line  7600)
* UTF-8 output encoding:                 Groff Options.     (line   751)
* v scaling unit:                        Measurements.      (line  4284)
* valid numeric expression:              Numeric Expressions.
                                                            (line  4522)
* value, incrementing without changing the register: Auto-increment.
                                                            (line  5403)
* variables in environment:              Environment.       (line   827)
* vee:                                   Page Geometry.     (line  4202)
* vee scaling unit (v):                  Measurements.      (line  4284)
* version number, major, register (.x):  Built-in Registers.
                                                            (line  5559)
* version number, minor, register (.y):  Built-in Registers.
                                                            (line  5563)
* vertical drawing position (nl):        Page Control.      (line  7240)
* vertical line drawing (\L):            Drawing Geometric Objects.
                                                            (line 10168)
* vertical line spacing register (.v):   Changing the Vertical Spacing.
                                                            (line  8543)
* vertical line spacing, changing (vs):  Changing the Vertical Spacing.
                                                            (line  8543)
* vertical line spacing, effective value: Changing the Vertical Spacing.
                                                            (line  8558)
* vertical motion (\v):                  Page Motions.      (line  9938)
* vertical motion quantum:               DESC File Format.  (line 13553)
* vertical page location, marking (mk):  Page Motions.      (line  9868)
* vertical page location, returning to marked (rt): Page Motions.
                                                            (line  9868)
* vertical position in diversion register (.d): Diversions. (line 11000)
* vertical position trap enable register (.vpt): Vertical Position Traps.
                                                            (line 10407)
* vertical position traps:               Vertical Position Traps.
                                                            (line 10399)
* vertical position traps, enabling (vpt): Vertical Position Traps.
                                                            (line 10407)
* vertical position, drawing (nl):       Page Control.      (line  7240)
* vertical resolution:                   DESC File Format.  (line 13553)
* vertical space unit (v):               Measurements.      (line  4284)
* vertical spacing:                      Page Geometry.     (line  4202)
* vertical spacing <1>:                  Manipulating Type Size and Vertical Spacing.
                                                            (line  8438)
* vertical spacing (introduction):       Basics.            (line  1136)
* warning categories:                    Warnings.          (line 12250)
* warning level (warn):                  Debugging.         (line 12227)
* warnings:                              Debugging.         (line 12222)
* warnings <1>:                          Warnings.          (line 12241)
* what is groff?:                        What Is groff?.    (line   264)
* while:                                 while.             (line  9327)
* while request, and font translations:  Selecting Fonts.   (line  7406)
* while request, and the ! operator:     Numeric Expressions.
                                                            (line  4365)
* while request, confusing with br:      while.             (line  9393)
* while request, operators to use with:  Operators in Conditionals.
                                                            (line  9027)
* widow:                                 Page Control.      (line  7203)
* widow <1>:                             Page Control.      (line  7222)
* width escape (\w):                     Page Motions.      (line 10027)
* width, of last glyph (.w):             Environments.      (line 11388)
* word space size register (.ss):        Manipulating Filling and Adjustment.
                                                            (line  5887)
* word, definition of:                   Filling.           (line  3581)
* write request, and copy mode:          I/O.               (line 11683)
* writec request, and copy mode:         I/O.               (line 11683)
* writem request, and copy mode:         I/O.               (line 11697)
* writing macros:                        Writing Macros.    (line  9410)
* writing to file (write, writec):       I/O.               (line 11683)
* year, current, register (year, yr):    Built-in Registers.
                                                            (line  5598)
* z scaling unit:                        Using Fractional Type Sizes.
                                                            (line  8604)
* zero-width printing (\z, \Z):          Page Motions.      (line 10102)
* zero-width printing (\z, \Z) <1>:      Page Motions.      (line 10107)
* zoom factor of a font (fzoom):         Selecting Fonts.   (line  7421)