code: def intersection(self, catalogue, output_fh):
          
          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)
          subquery = self._get_intersection_subquery(labels)
          query = constants.SELECT_INTERSECT_SQL.format(label_placeholders,
                                                        subquery)
          parameters = labels + labels
          self._logger.info('Running intersection query')
          self._logger.debug('Query: {}\nLabels: {}'.format(query, labels))
          self._log_query_plan(query, parameters)
          cursor = self._conn.execute(query, parameters)
          return self._csv(cursor, constants.QUERY_FIELDNAMES, output_fh)
docs: Returns `output_fh` populated with CSV results giving the
          intersection in n-grams of the witnesses of labelled sets of
          works in `catalogue`.
  
          :param catalogue: catalogue matching filenames to labels
          :type catalogue: `Catalogue`
          :param output_fh: object to output results to
          :type output_fh: file-like object
          :rtype: file-like object