code: def _frequency_order_transform(sets):
      logging.debug("Applying frequency order transform on tokens...")
      counts = reversed(Counter(token for s in sets for token in s).most_common())
      order = dict((token, i) for i, (token, _) in enumerate(counts))
      sets = [np.sort([order[token] for token in s]) for s in sets]
      logging.debug("Done applying frequency order.")
      return sets, order