// Set intersection supporting std::unordered_set.
template <typename Set, typename Key = typename Set::value_type>
static inline Set set_intersection(const Set& lhs, const Set& rhs) {
    if (lhs.size() <= rhs.size()) {
        Set iset;
        for (const Key& key : lhs) {
            if (rhs.count(key) > 0) {
                iset.insert(key);
            }
        }
        return std::move(iset);
    } else {
        return set_intersection(rhs, lhs);
    }
}