int num_loop_vars = params->ninputs;

  mutex_lock l(parent->mu);

  // 'cond_fn' copies the cond graph into the parent graph.
  tensorflow::ops::CondGraphBuilderFn cond_fn =
      [params, parent](const tensorflow::Scope& scope,
                       const std::vector<tensorflow::Output>& inputs,
                       tensorflow::Output* output) {