I would like to create a macro that defines a display formula, and then
redefine the same macro such that the redefinition includes the original
display formula followed by another display formula.  Here is my attempt.

\def\foo{\startformula a = b \stopformula} \foo
\edef\foo{\foo \startformula c = d \stopformula} \foo

This fails, and I would appreciate help in getting this to work.

Thanks,

Troy Henderson