/// Retrieves underlying value from a TaggedValue with type DICT.
  impl::Dict& dict() { return *get<impl::DictPtr>(); }
  /// Retrieves underlying value from TaggedValues with type DICT.
  const impl::Dict& dict() const { return *get<impl::DictPtr>(); }

  /// Retrieves underlying value from a TaggedValue with type FUNC.
  impl::Func func() const { return get<impl::Func>(); }