4.3BSD-Tahoe/usr/lib/learn/eqn/L7.1c

#once #create message
.ND
.tr %$
.EQ
delim $$
.EN
.LP
It is also quite permissible to leave out
either the "from" part or the "to" part
of a from-to construction.  For example,
modify "Example" so its output looks like this,
then type "ready".
.pl 1
#once #create Ref
.LP
.EQ
lim from {x-> pi /2} ( tan~x) sup {sin~2x}~=~1
.EN
.pl 1
#once #create Example
.LP
.EQ
lim from xxx ( tan~x) sup {sin~2x}~=~1
.EN
.pl 1
#
#once neqn Ref | nroff >X1 &
#once neqn message Ref | nroff -T$term %s/tinyms -
#user
neqn Example | nroff >X2
#cmp X1 X2
#log
#next
7.1d 10
7.2c 5