docs: def _list_ids(path_to_data):
      """List raw data IDs grouped by symbol ID from a pickle file
         ``path_to_data``."""
      loaded = pickle.load(open(path_to_data, "rb"))
      raw_datasets = loaded['handwriting_datasets']
      raw_ids = {}
      for raw_dataset in raw_datasets:
          raw_data_id = raw_dataset['handwriting'].raw_data_id
          if raw_dataset['formula_id'] not in raw_ids:
              raw_ids[raw_dataset['formula_id']] = [raw_data_id]
          else:
              raw_ids[raw_dataset['formula_id']].append(raw_data_id)
      for symbol_id in sorted(raw_ids):
          print("%i: %s" % (symbol_id, sorted(raw_ids[symbol_id])))