  auto operator()(Args && ... args) const
                  -> decltype(format(str, std::forward<Args>(args)...)) {
    return format(str, std::forward<Args>(args)...);
  }
  auto operator()(Args && ... args) const
                  -> decltype(format(str, std::forward<Args>(args)...)) {
    return format(str, std::forward<Args>(args)...);
  }