f a
        given unit and the difference between the starting position and the
        ending position is not coprime with the greatest common denominator
        between the interval and the base. For example, with a FREQ of hourly
        starting at 17:00 and an interval of 4, the only valid values for
        BYHOUR would be {21, 1, 5, 9, 13, 17}, because 4 and 24 are not
        coprime.

        :param start:
            Specifies the starting position.
        :param byxxx:
            An iterable containing the list of allowed values.
        :param base:
            The largest allowable value for the specified frequency (e.g.
            24 hours, 60 minutes).

        This does not preserve the type of the iterable, returning a set, since
        the values should be unique and the order is irrelevant, this will
        speed up later lookups.

        In the event of an empty set, raises a :exception:`ValueError`, as this
        results in an empty rrule.
        r