Sometimes you will wish to generate a set of aligned equations
but refer to each of them individually.
In this case you will wish to be able to number them independently.
PLAIN TEX provides the control sequence
eqaligno
to handle this eventuality.
The command
eqaligno works like
eqalign
except that each equation line consists of three
parts (or templates) separated by the character &.
The third template is where you put the appropriate label for
that line of the set of equations.
If you wish to number each equation line sequentially without
generating a name, then all you have to do is type
$$
eqaligno { junk & more junk &
eq
cr } $$
If, however, you wish to name each line separately
you may not use the macro
eqn.
It will not work and will give you a peculiar message of the form
you cannot use
eqno inside
eqalign.
To name individual lines of aligned equations the correct macro
to use is
eqnalign {
name } as the
entry for the third template followed by the command
cr.
The same command will work in
leqaligno which works
in exactly the same way as
eqalign but puts the contents
of the third template to the left of each equation instead of to
the right.
In general,
leqaligno causes more difficulties than
eqaligno and you should read about it in the TEXbook
before using it.