#include <iostream>
#include <string>
#include <unordered_map>

template <typename Type>
concept Addable =
    requires(Type lh, Type rh)
    {
        lh + rh;
    };

template <typename Type>
concept Addable2 =
    Addable<Type>
    and
    requires(Type lh, Type rh)
    {
        lh += rh;
    };


template<Addable Type>
Type add(Type const &x, Type const &y)
{
    return x + y;
}

template<typename Type>
Type add2(Type const &x, Type const &y)
{
    return x + y;
}

using namespace std;

int main()
{
    add("one"s, "two"s);
//    add(unordered_map<int, int>{}, unordered_map<int, int>{});
}