% 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