code: def _series_feedback(series, out_port, in_port):
      
      series_s = series.series_inverse().series_inverse()
      if series_s == series:
          raise CannotSimplify()
      return series_s.feedback(out_port=out_port, in_port=in_port)
docs: Invert a series self-feedback twice to get rid of unnecessary
      permutations.