Plan 9 from Bell Labs’s /usr/web/sources/patch/applied/cc-varadic-macros/readme
this patch enables support for c99 varadic macros for ?c. the best example of use would be #define debug(...) fprint(2, __VA_ARGS__) or #define debug(...) or #define xyzdebug(...) if(xyz->debug) fprint(2, ___VA_ARGS__)