template<class T>
struct is_rule<T, void_t<decltype(std::declval<system::result<typename T::value_type>&>() =
                std::declval<T const&>().parse(
                    std::declval<char const*&>(),
                    std::declval<char const*>()))>>
    : std::is_nothrow_copy_constructible<T>;