Granted I haven't tried to get it through review, but for things where the fast version is complicated I usually end up with static bool set_t_contains_fas

Granted I haven't tried to get it through review, but for things where the fast version is complicated I usually end up with static bool set_t_contains_fast(set_t x, uint64_t e); static bool set_t_contains_simple(set_t x, uint64_t e); bool set_t_contains(set_t x, uint64_t e) { bool f = set_t_contains_fast(x, e); assert(set_t_contains_simple(x, e) == f); return f; } sometimes with a CONTRACTS macro controlling it instead of an assert.