//           can be null. The names used for function inputs are normalized
//           names of the operations (usually placeholders) pointed to by
//           `inputs`. These operation names should start with a letter.
//           Normalization will convert all letters to lowercase and
//           non-alphanumeric characters to '_' to make resulting names match
//           the "[a-z][a-z0-9_]*" pattern for operation argument names.

    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.