docs: def _add_text_ngrams(self, witness, minimum, maximum):
          """Adds n-gram data from `witness` to the data store.
  
          :param witness: witness to get n-grams from
          :type witness: `WitnessText`
          :param minimum: minimum n-gram size
          :type minimum: `int`
          :param maximum: maximum n-gram size
          :type maximum: `int`
  
          """
          text_id = self._get_text_id(witness)
          self._logger.info('Adding n-grams ({} <= n <= {}) for {}'.format(
              minimum, maximum, witness.get_filename()))
          skip_sizes = []
          for size in range(minimum, maximum + 1):
              if self._has_ngrams(text_id, size):
                  self._logger.info(
                      '{}-grams are already in the database'.format(size))
                  skip_sizes.append(size)
          for size, ngrams in witness.get_ngrams(minimum, maximum, skip_sizes):
              self._add_text_size_ngrams(text_id, size, ngrams)