@abc.abstractmethod<EOL><INDENT>def update_event(self, event_form):<DEDENT>
pass<EOL>
Updates an existing event.

        :param event_form: the form containing the elements to be updated
        :type event_form: ``osid.calendaring.EventForm``
        :raise: ``IllegalState`` -- ``event_form`` already used in an update transaction
        :raise: ``InvalidArgument`` -- one or more of the form elements is invalid
        :raise: ``NullArgument`` -- ``event_form`` is ``null``
        :raise: ``OperationFailed`` -- unable to complete request
        :raise: ``PermissionDenied`` -- authorization failure
        :raise: ``Unsupported`` -- ``event_form`` did not originate from ``get_event_form_for_update()``

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