Clever use of `__VA_ARGS__`! Does gcc remove `_a` as well? Well, may be it was just my implementation:

  #define mkmacro(func, ...) mkmacro_(func, VA_NUM_ARGS(__VA_ARGS__))
  #define mkmacro_(func, nargs) mkmacro__(func, nargs)
  #define mkmacro__(func, nargs) func ## nargs
  #define DO(...) mkmacro(DO, __VA_ARGS__)(__VA_ARGS__)
  #define DO1(n) for(I i=0;i<(n); i++)
  #define DO2(m,n) DO1(m) for(I j=0;j<(n);j++)
  #define DO3(l,m,n) DO2(l,m) for(I k=0;k<(n);k++)