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+% This file is public domain.
+%
+% Test symbolic equation references.
+% 
+\ifx\eplain\undefined \input eplain \fi
+
+\loggingall
+\leftdisplays
+
+An equation with a number in its name:
+$$ a+b=1\eqdef{line1}$$
+\line{did we mess up plain's line in equation \eqref{line1}?\hfil}
+
+\bye
+
+The first equation:
+$$x + y = 1 \eqdef{first}$$
+
+In equation \eqref{first}, we said that $x+y=1$.
+Now, in equation \eqref{second}, we say that $a+b=2$.
+
+$$a + b = 2 \eqdef{second}$$
+
+The next equation is unlabeled in the output, but we can still refer to
+it.
+$$c + d = 3 \eqdefn{*}$$
+
+And here's an equation whose text references another equation, namely,
+\eqref{first}:
+$$c=3\eqdef[\eqrefn{first}*]{first-star}$$
+And a reference to it: \eqref{first-star}.
+
+Here is the first of a group of equations: \eqdefn{group}
+$$a = 1\eqsubdef{group-1}$$
+and the invisible second, then the third of the group \eqsubdefn{group-invis}:
+$$b = 2\eqsubdef{group-2}$$
+We refer to the group as \eqref{group}, to the first as
+\eqref{group-1}, the third as \eqref{group-2}, and the invisible second
+as \eqref{group-invis}.
+
+Here's an equation labeled strangely:
+$$a=1\eqdef[\rm strange*]{foo}$$
+And let's refer to \eqref{foo}.
+
+Let's do subequations off that:
+$$b=2\eqsubdef{foo-1}$$
+And refer to it: \eqref{foo-1}.
+
+% We can't use \count255, since #1 might involve contortions which
+% trample it.
+\newcount\subrefcount
+\def\eqsubreftext#1#2{%
+  \subrefcount = #2
+  \advance\subrefcount by 96
+  #1\char\subrefcount
+}
+
+Let's try another group, with a different labelling scheme.  Here is the
+first of this group2: \eqdefn{group2} 
+$$a = 1\eqsubdef{group2-1}$$
+and the second of the group2 \eqsubdefn{group2-invis}:
+$$b = 2\eqsubdef{group2-2}$$
+We refer to the group2 as \eqref{group2}, to the first as
+\eqref{group2-1}, the third as \eqref{group2-2}, and the invisible second
+as \eqref{group2-invis}.
+
+Here is one done with displaylines:
+% The \hfill's and \llap make the equation numbers come out in the right
+% place if we are not doing \leftdisplays.  (See the TeXbook, p.194.)
+%$$\displaylines{
+%  \hfill x=1\hfill\llap{\eqdef{displayline-1}}\cr
+%  \hfill y=2\hfill\llap{\eqdef{displayline-2}}\cr
+%}$$
+$$\displaylines{
+   x=1\eqdef{displayline-1}\cr
+   y=2\eqdef{displayline-2}\cr
+}$$
+
+Let's refer to each of the equations in the displaylines:
+\eqref{displayline-1}, \eqref{displayline-2}.
+
+And one with eqalignno:
+$$
+\eqalignno{
+   a+b&=c&\eqdef{eqalign-1}\cr
+   d+e&=f&\eqdef{eqalign-2}\cr
+}
+$$
+
+And one with leqalignno, and indented more:
+{\leftdisplayindent = 1in
+$$\leqalignno{
+   a+b  &=c&\eqdef{leqalign-1}\cr
+   dt+eg&=f&\eqdef{leqalign-2}\cr
+}$$
+}
+
+And now let's refer to those: \eqref{eqalign-1}, \eqref{eqalign-2},
+\eqref{leqalign-1}, \eqref{leqalign-2}.
+
+
+
+\def\eqprint#1{[\rm A.#1]}
+
+From now on, all equation labels should be in brackets and preceded
+by `A.'.  In \eqref{*}, we said that $c+d=3$.  And in equation
+\eqref{undefined}, we say that $1+1=2$.
+
+\eqdefn{*x}
+\eqdefn{*y}
+
+We defined equation \eqref{*y}, although we didn't give an equation for
+it.
+This is a forward reference to equation \eqref{forward}.
+
+$$e + f = 4 \eqdef{forward}$$
+
+That equation above defines \eqref{forward}.
+
+
+\newcount\sectionnum \sectionnum = 1
+\def\eqconstruct#1{\the\sectionnum.#1}
+
+And now equation numbers should have a section number (which we start at
+one) preceding them.
+
+$$a+b=1 \eqdef{s-ab}$$
+
+That equation is \eqref{s-ab}.  The one we'll define in the next section
+is \eqref{s-cd}.
+
+
+\advance\sectionnum by 1
+The equation in the previous section was \eqref{s-ab}.  Let's define one
+in this section:
+
+$$c+d=3 \eqdef{s-cd}$$
+
+And refer to it: \eqref{s-cd}.
+
+\bye