@abc.abstractmethod<EOL><INDENT>def get_weekly_interval(self):<DEDENT>
return<EOL>
Gets the number of weeks of the interval.

        1 is every week. 2 is every other week. -1 is every week back in
        time.

        :return: ``the week interval``
        :rtype: ``integer``
        :raise: ``IllegalState`` -- ``has_weekdly_interval()`` is ``false`` or ``has_weekof_month_interval()`` is
        ``true``

        *compliance: mandatory -- This method must be implemented.*