/// 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>(); }

    OutList(o_, x->begin(), x->end(), '[', ']');
    return o_;
  }
  std::ostream& operator()(const TuplePtr& x) {
    OutList(o_, x->begin(), x->end(), '(', ')');
    return o_;
  }
  std::ostream& operator()(const DictPtr& x) {
    o_ << *x;
    return o_;
  }
  std::ostream& operator()(const Capsule& x) {
    o_ << "Capsule(" << x.get() << ")";
    return o_;
  }
  std::ostream& operator()(const Func& x) {
    o_ << "Func";