def METHOD_NAME(signal):
    """Generate a sequence number for external identifier in CityControl."""
    roundtrip_count = CityControlRoundtrip.objects.filter(_signal=signal).count()
    if not roundtrip_count < MAX_ROUND_TRIPS:  # check not sent too often
        msg = f'Signal {signal.sia_id} was sent to SigmaxCityControl too often.'
        raise SentTooManyTimesError(msg)
    return '{0:02d}'.format(roundtrip_count + 1)  # start counting at one