class ClientSession::Impl {
 private:
  friend class ClientSession;

  Impl(Session* session, std::shared_ptr<Graph> graph)
      : session_(session), graph_(std::move(graph)) {}

  static SessionOptions MakeDefaultSessionOptions(const string& target);
  Status MaybeExtendGraph() const;

  std::unique_ptr<Session> session_;
  std::shared_ptr<Graph> graph_;

  mutable mutex mu_;