// already exists.
  virtual absl::Status AddFunctionDef(const FunctionDef& fdef) = 0;

  // Notifies about the function removal.
  virtual absl::Status AddRemoveFunctionNotifier(
      const string& func, std::function<void()> notifier) = 0;

  // Same as `AddFunctionDef`, but additionally saves the `stack_traces` under