@abc.abstractmethod<EOL><INDENT>def add_schedule(self, recurring_event_id, schedule_id):<DEDENT>
return<EOL>
Adds a schedule to the given recurring event.

        :param recurring_event_id: a recurring event ``Id``
        :type recurring_event_id: ``osid.id.Id``
        :param schedule_id: a schedule ``Id``
        :type schedule_id: ``osid.id.Id``
        :return: the ``Id`` of the schedule
        :rtype: ``osid.id.Id``
        :raise: ``AlreadyExists`` -- schedule is already part of recurring event
        :raise: ``NotFound`` -- ``recurring_event_id`` or ``schedule_id`` is not found
        :raise: ``NullArgument`` -- ``recurring_event_id`` or ``schedule_id`` is ``null``
        :raise: ``OperationFailed`` -- unable to complete request
        :raise: ``PermissionDenied`` -- authorization failure

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