def null_index_check(layer_id, word_id, neuron_id):
  """Checks if some of the indices should be nulled.

  Args:
    layer_id: The index for the layer.
    word_id: The index for the word.
    neuron_id: The index for the neuron.

  Returns:
    lid: Checked layer id.
    wid: Checked word id.
    nid: Checked neuron id.
  """
  lid = layer_id if layer_id != -1 else None
  wid = word_id if word_id != -1 else None
  nid = neuron_id if neuron_id != -1 else None
  return lid, wid, nid