[NTG-context] Nested Macro

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