inline int compare(const Subtype& other) const { return m_id - other.id(); }
handle(const Subtype& other) { m_id = other.id(); }
int64_t compare(const Subtype& other) const { return id_ - other.id(); }
friend std::string to_string(const Subtype& x) { return std::to_string(x.id()); }
handle(const Subtype& other) { id_ = other.id(); }