// For LLVM style RTTI.
  static bool classof(const AbstractContext* ptr) {
    return ptr->getKind() == kTape;
  }
  ~TapeContext() override;

 private:
  AbstractContext* parent_ctx_;  // Not owned.
  Tape* tape_;
  const GradientRegistry& registry_;
};
}  // namespace gradients
}  // namespace tensorflow