:param calres: a zobjects.CalendarResource
        :param attrs:    a dictionary of attributes to set ({key:value,...})

def modify_calendar_resource(self, calres, attrs):
        """
        :param calres: a zobjects.CalendarResource
        :param attrs:    a dictionary of attributes to set ({key:value,...})
        """
        attrs = [{'n': k, '_content': v} for k, v in attrs.items()]
        self.request('ModifyCalendarResource', {
            'id': self._get_or_fetch_id(
                calres, self.get_calendar_resource),
            'a': attrs
        })