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+#once #create message
+.ND
+.tr %$
+.EQ
+delim $$
+.EN
+.LP
+The next topic goes back to the problem of formatting
+equations on the page.  Sometimes it is desirable to
+line up one part of an equation with some part
+of a previous equation - for example, equals signs
+are often lined up like this:
+.EQ I
+x mark = y sub i
+.EN
+.EQ I
+lineup = z sub i
+.EN
+.EQ I
+x sub i + y sub i lineup = z sub i
+.EN
+
+To achieve this, you have to do several things.
+(1) tell neqn to remember the place where things are
+to be lined up, which is done by saying
+"mark" just before the thing you want remembered:
+  .EQ I
+  x mark = y sub i
+  .EN
+.br
+(2) Then, in successive equations, telling neqn
+to line up something on the previous mark, with
+"lineup":
+   .EQ I
+   lineup = z sub i
+   .EN
+.br
+(3) You ____have __to use either ".EQ I" or ".EQ L";
+you can't line things up in a centered equation.
+
+For practice, modify "Example" so the
+equations are lined up on the equals signs, then
+type "ready".
+.pl 1
+#once #create Ref
+.LP
+A test of mark and lineup:
+.EQ I
+x sub i ~ mark =~ 1
+.EN
+.EQ I
+x sub i + y sub i ~ lineup = ~2
+.EN
+.pl 1
+#once #create Example
+.LP
+A test of mark and lineup:
+.EQ I
+x sub i ~ =~ 1
+.EN
+.EQ I
+x sub i + y sub i ~ =~ 2
+.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
+#fail
+Don't forget that tilde is a real character;
+put the mark and lineup after it.
+Remember .EQ I.
+#log
+#next
+10.1b 10