#define NOTHING
#define DEFD(a) ((0 == a)?(NOTHING):(1))

#define PRIMITIVE_CAT(a, ...) a ## __VA_ARGS__

#define COMPL(b) PRIMITIVE_CAT(COMPL_, b)
#define COMPL_0 1
#define COMPL_1 0


#define CHECK_N(x, n, ...) n
#define CHECK(...) CHECK_N(__VA_ARGS__, 0,)
#define PROBE(x) x, 1,

#define IS_PAREN(x) CHECK(IS_PAREN_PROBE x)
#define IS_PAREN_PROBE(...) PROBE(~)

#define NOT(x) CHECK(PRIMITIVE_CAT(NOT_, x))
#define NOT_0 PROBE(~)

#define BOOL(x) COMPL(NOT(x))

#define IIF(c) PRIMITIVE_CAT(IIF_, c)
#define IIF_0(t, ...) __VA_ARGS__
#define IIF_1(t, ...) t

#define IF(c) IIF(BOOL(c))

#define EAT(...)
#define EXPAND(...) __VA_ARGS__
#define WHEN(c) IF(c)(EXPAND, EAT)

#define SQUARE(b,a)     (WHEN(a)((b)*(b)))

printf("---%d",SQUARE(2));

printf("---%d",SQUARE(0));