select!()
Equivalent to Hash#keep_if, but returns nil if no changes were made.
VALUE rb_hash_select_bang(VALUE hash) { st_index_t n; RETURN_ENUMERATOR(hash, 0, 0); rb_hash_modify(hash); if (!RHASH(hash)->ntbl) return Qnil; n = RHASH(hash)->ntbl->num_entries; rb_hash_foreach(hash, keep_if_i, hash); if (n == RHASH(hash)->ntbl->num_entries) return Qnil; return hash; }