def _series_feedback(series, out_port, in_port):
    """Invert a series self-feedback twice to get rid of unnecessary
    permutations."""
    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)
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 )
Invert a series self-feedback twice to get rid of unnecessary
    permutations.