docs: def _flatten(self, iterator, **filter_options):
          '''
          iterator here gives as lists of tuples. Method flattens the structure
          to a single list of tuples.
          '''
          resp = list()
          for entry in iterator:
              for tup in entry:
                  if self._matches_filter(tup, **filter_options):
                      resp.append(tup)
          return resp