Method operator<

src/fl/stl/multi_set.h:373–379

    }

    bool operator<(const MultiSetTree& other) const {
        for (auto it1 = begin(), it2 = other.begin(); it1 != end() && it2 != other.end(); ++it1, ++it2) {
            if (*it1 < *it2) return true;
            if (*it2 < *it1) return false;
        }
        return size() < other.size();
    }

    bool operator<=(const MultiSetTree& other) const {
        return *this < other || *this == other;