docs: def _sort_labels(label_data):
          """Returns the labels in `label_data` sorted in descending order
          according to the 'size' (total token count) of their referent
          corpora.
  
          :param label_data: labels (with their token counts) to sort
          :type: `dict`
          :rtype: `list`
  
          """
          labels = list(label_data)
          labels.sort(key=label_data.get, reverse=True)
          return labels