ram byxxx:
            The `BYXXX` set, which should have been generated by
            `rrule._construct_byset`, or something else which checks that a
            valid rule is present.
        :param base:
            The largest allowable value for the specified frequency (e.g.
            24 hours, 60 minutes).

        If a valid value is not found after `base` iterations (the maximum
        number before the sequence would start to repeat), this raises a
        :exception:`ValueError`, as no valid values were found.

        This returns a tuple of `divmod(n*interval, base)`, where `n` is the
        smallest number of `interval` repetitions until the next specified
        value in `byxxx` is found.
        r