__code pop2SingleLinkedStack(struct SingleLinkedStack* stack, __code next(union Data* data, union Data* data1, ...)) {
    if (stack->top) {
        data = stack->top->data;
        stack->top = stack->top->next;
    } else {
        data = NULL;
    }
    if (stack->top) {
        data1 = stack->top->data;
        stack->top = stack->top->next;
    } else {
        data1 = NULL;
    }
    goto next(data, data1, ...);
}