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,109 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;110 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.