docs: def diff(self, catalogue, tokenizer, output_fh):
          """Returns `output_fh` populated with CSV results giving the n-grams
          that are unique to the witnesses of each labelled set of works
          in `catalogue`.
  
          Note that this is not the same as the symmetric difference of
          these sets, except in the case where there are only two
          labels.
  
          :param catalogue: catalogue matching filenames to labels
          :type catalogue: `Catalogue`
          :param tokenizer: tokenizer for the n-grams
          :type tokenizer: `Tokenizer`
          :param output_fh: object to output results to
          :type output_fh: file-like object
          :rtype: file-like object
  
          """
          labels = self._sort_labels(self._set_labels(catalogue))
          if len(labels) < 2:
              raise MalformedQueryError(
                  constants.INSUFFICIENT_LABELS_QUERY_ERROR)
          label_placeholders = self._get_placeholders(labels)
          query = constants.SELECT_DIFF_SQL.format(label_placeholders,
                                                   label_placeholders)
          parameters = labels + labels
          self._logger.info('Running diff query')
          self._logger.debug('Query: {}\nLabels: {}'.format(query, labels))
          self._log_query_plan(query, parameters)
          cursor = self._conn.execute(query, parameters)
          return self._diff(cursor, tokenizer, output_fh)