message: das.cli: Make `extend` const.
diff: @@ -251,7 +251,7 @@ namespace das
  template <typename... NewArgs>
  auto
  - extend(NewArgs&& ... args)
  + extend(NewArgs&& ... args) const
  {
  return Prototype<NewArgs..., Formal...>
  (this->defaults.extend(std::forward<NewArgs>(args)...));
  