const std::function<void(BackwardFunction*)>& backward_function_deleter) {
  if (call_state_.top().backward_tape != nullptr) {
    // If backward_tape is not null, then this call to Accumulate is the result
    // of a still-active call to Accumulate which is running operations. We
    // forward these operations to backward_tape so the outer Accumulate call
    // can do its work.