@abc.abstractmethod<EOL><INDENT>def get_start_reference_event(self):<DEDENT>
return<EOL>
Gets the ``Event`` to which the start of this event is offset.

        :return: the relative event
        :rtype: ``osid.calendaring.Event``
        :raise: ``IllegalState`` -- ``has_fixed_start_time()`` is ``true``
        :raise: ``OperationFailed`` -- unable to complete request

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