for (const auto& e : v) {
      if (i != 0) {
        ss << ", ";
      }
      ss << e.ToString();
      ++i;
    }
    ss << "}";
    return ss.str();
  }

  void VerifyFDefNodes(const tensorflow::FunctionDef& fdef,
                       const std::unordered_set<string>& nodes) {
    ASSERT_EQ(nodes.size(), fdef.node_def_size())
        << "Got unexpected number of nodes. Expected: ["