(eplain.info)Equation references

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4.11 Equation references
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Instead of referring to pages, it's most useful if equation labels refer
to equation numbers. Therefore, Eplain reserves a '\count' register,
'\eqnumber', for the current equation number, and increments it at each
numbered equation.

   Here are the commands to define equation labels and then refer to
them:

'\eqdef{LABEL}'
     This defines LABEL to be the current value of '\eqnumber', and, if
     the current context is not inner, then produces a '\eqnum' command
     (Note: Displays). (The condition makes it possible to use
     '\eqdef' in an '\eqalignno' construction, for example.) The text of
     the equation number is produced using '\eqprint'. Note: Formatting
     equation references.

     If LABEL is empty, you still get an equation number (although
     naturally you can't reliably refer to it). This is useful if you
     want to put numbers on all equations in your document, and you
     don't want to think up unique labels.

     To refer to the last equation with the empty label, you just use
     the empty label in one of the equation reference macros (see
     below). This can be handy when you want to refer to an equation
     shortly after its definition, say, in the sentence following the
     displayed equation, and do not intend to refer to the equation
     later. But use this trick with extreme caution: if later you change
     the text and insert another empty definition between the original
     definition and the reference, the reference will start to refer to
     the new empty-labeled equation.

'\eqdefn{LABEL}'
     This is like '\eqdef', except it always omits the '\eqnum' command.
     It can therefore be used in places where '\eqdef' can't; for
     example, in a non-displayed equation. The text of the equation
     number is not produced, so you can also use it in the (admittedly
     unusual) circumstance when you want to define an equation label but
     not print that label.

'\eqref{LABEL}'
     This produces a formatted reference to LABEL. If LABEL is undefined
     (perhaps because it is a forward reference), it just produces the
     text of the label itself. Otherwise, it calls '\eqprint'.

'\eqrefn{LABEL}'
     This produces the cross-reference text for LABEL. That is, it is
     like '\eqref', except it doesn't call '\eqprint'.

   Equation labels can contain the same characters that are valid in
general cross-references.

   Eplain can create hypertext links for the equation references (Note:
Equation reference hyperlinks).

Formatting equation references

Subequation references