code: def _compute_distance_fast(self):
          
          d, m = edit_distance(self.seq1, self.seq2,
                               action_function=self.action_function,
                               test=self.test)
          if self.dist:
              assert d == self.dist
          if self._matches:
              assert m == self._matches
          self.dist = d
          self._matches = m
docs: Calls edit_distance, and asserts that if we already have values for
          matches and distance, that they match.