Oddballs: std::multiset<T> and std::multimap<K, V>

In STL-speak, a "set" is an ordered, deduplicated collection of elements. So naturally, a "multiset" is an ordered, non-deduplicated collection of elements! Its memory layout is exactly the same as the layout of std::set; only its invariants are different. Notice in the following diagram that std::multiset allows two elements with value 42:

std::multiset<T, Cmp> behaves just like std::set<T, Cmp>, except that it can store duplicate elements. The same goes for std::multimap<K, V, Cmp>:

    std::multimap<std::string, std::string> mm;
    mm.emplace("hello", "world");
    mm.emplace("quick", "brown");
    mm.emplace("hello", "dolly");
    assert(mm.size() == 3);

    // Key-value pairs are stored in sorted order.
    // Pairs with identical keys are guaranteed to be
    // stored in the order in which they were inserted.
    auto it = mm.begin();
    using Pair = decltype(mm)::value_type;
    assert(*(it++...