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diff --git a/contrib/chem/examples/122/ch6b_dna.chem b/contrib/chem/examples/122/ch6b_dna.chem
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+ch6b_dna.chem:
+.br
+.cstart
+
+# Example file for 'chem':
+
+# This originates from Computing Science Technical Report No. 122
+#   CHEM - A Program for Typesetting Chemical Diagrams: User Manual
+#   by Jon L. Bentley, Lynn W. Jelinski, Brian W. Kernighan
+#   <http://cm.bell-labs.com/cm/cs/cstr/122.ps.gz>.
+
+# Copyright (C) 2006-2020 Free Software Foundation, Inc.
+
+# This file is part of 'chem', which is part of 'groff'.
+
+# 'groff' is free software; you can redistribute it and/or modify it
+# under the terms of the GNU General Public License (GPL) version 2 as
+# published by the Free Software Foundation.
+
+# 'groff' is distributed in the hope that it will be useful, but
+# WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+# General Public License for more details.
+
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+# The GPL2 license text is available in the internet at
+# <http://www.gnu.org/licenses/gpl-2.0.html>.
+
+########################################################################
+
+P: [
+R1:  flatring pointing up put N at 1 put N at 4 double 5,1
+     bond -135 from R1.V4 ; BP
+     "deoxyribose" rjust with .e at BP.w
+R2:  ring6 put N at 2 put N at 4 double 1,2 3,4 5,6 with .V6 at R1.V2
+     pic Conn: R2.V2.ne #because naming is too restricted in pic
+     bond up from R2.V1 ; N
+     bond -60 from N ; H
+     bond 60 from N ; H
+]
+     # thymine
+Q: [
+R3:  ring6 put N at 3 put N at 5 double 1,2
+     bond up from R3.V1 ; CH3
+     bond 120 from R3.V3 ; BP
+     "deoxyribose" ljust with .w at BP.e
+     double bond down from R3.V4 ; O
+     double bond -60 from R3.V6 ; O
+     bond -120 from R3.V5 ; H
+] with .O at P.H + (.3,.3)
+     bond from Q.O.sw to P.H.ne dotted
+     bond from Q.H.sw to P.Conn dotted
+
+# Local Variables:
+# mode: Nroff
+# End:
+.cend