typedef struct Queue<Impl>{
        union Data* queue;
        union Data* data;
        __code whenEmpty(...);
        __code clear(Impl* queue, __code next(...));
        __code put(Impl* queue, union Data* data, __code next(...));
        __code take(Impl* queue, __code next(union Data*, ...));
        __code isEmpty(Impl* queue, __code next(...), __code whenEmpty(...));
        __code next(...);
} Stack;