Neg(oper, graph, s);
  ASSERT_EQ(TF_OK, TF_GetCode(s)) << TF_Message(s);
  ASSERT_TRUE(TF_GraphOperationByName(graph, "neg") != nullptr);

  // Export to a GraphDef.
  TF_Buffer* graph_def = TF_NewBuffer();
  TF_GraphToGraphDef(graph, graph_def, s);
  ASSERT_EQ(TF_OK, TF_GetCode(s)) << TF_Message(s);

  // Import it, with a prefix, in a fresh graph.
  TF_DeleteGraph(graph);
  graph = TF_NewGraph();

    // to still check.
    if (node != nullptr) return ToOperation(node);
    *pos += 1;
  }

  // No more nodes.
  return nullptr;
}

void TF_GraphToGraphDef(TF_Graph* graph, TF_Buffer* output_graph_def,
                        TF_Status* status) {
  GraphDef def;
  {
    mutex_lock l(graph->mu);
    graph->graph.ToGraphDef(&def);
  }