(eplain.info)Subequation references

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4.11.2 Subequation references
-----------------------------

Eplain also provides for one level of substructure for equations. That
is, you might want to define a related group of equations with numbers
like '2.1' and '2.2', and then be able to refer to the group as a whole:
"... in the system of equations (2)...".

   The commands to do this are '\eqsubdef' and '\eqsubdefn'. They take
one LABEL argument like their counterparts above, and generally behave
in the same way. The difference is in how they construct the equation
number: instead of using just '\eqnumber', they also use another
counter, '\subeqnumber'. This counter is advanced by one at every
'\eqsubdef' or '\eqsubdefn', and reset to zero at every '\eqdef' or
'\eqdefn'.

   You use '\eqref' to refer to subequations as well as main equations.

   To put the two together to construct the text that the label will
produce, they use a macro '\eqsubreftext'. This macros takes two
arguments, the "main" equation number (which, because the equation label
can be defined as arbitrary text, as described in the previous section,
might be anything at all) and the "sub" equation number (which is always
just a number). Eplain's default definition just puts a period between
them:

     \def\eqsubreftext#1#2{#1.#2}%

You can redefine '\eqsubreftext' to print however you like. For example,
this definition makes the labels print as '2a', '2b', and so on.

     \newcount\subref
     \def\eqsubreftext#1#2{%
       \subref = #2           % The space stops a <number>.
       \advance\subref by 96  % `a' is character code 97.
       #1\char\subref
     }

Sadly, we must define a new count register, '\subref', instead of using
the scratch count register '\count255', because '#1' might include other
macro calls which use '\count255'.