message: das.named: Add `extend`.
diff: @@ -222,6 +222,16 @@ namespace das
  template <typename T>
  using default_for = DefaultFor<is_effective<T>::value, T>;
  +
  + template <typename ... Args>
  + auto
  + extend(Args&& ... args) const
  + {
  + return DefaultStore<Args ..., Formal ...>(
  + std::forward<Args>(args)...,
  + static_cast<DefaultFor<is_effective<Formal>::value, Formal> const&>(
  + *this)...);
  + }
  };
  /*----------.
  @@ -239,6 +249,14 @@ namespace das
  : defaults(std::move(d))
  {}
  + template <typename... NewArgs>
  + auto
  + extend(NewArgs&& ... args)
  + {
  + return Prototype<NewArgs..., Formal...>
  + (this->defaults.extend(std::forward<NewArgs>(args)...));
  + }
  +
  template <typename Sequence>
  struct Call;
  