template <typename It1, typename It2, typename Out>
Out mystd::set_difference(It1 first1, It1 last1,
                          It2 first2, It2 last2, Out out) {
    while (first1 != last1) {
        if (first2 == last2) {
            return std::copy(first1, last1, out);
        }
        if (*first1 < *first2) {
            *out++ = *first1++;
        } else {
            if (*first2 >= *first1) {
                ++first1;
            }
            ++first2;
        }
    }
    return out;
}