'Get a list of Events for a given topic that occurred before (and
including) the events in event_list. Return a list of max size `limit`
Args:
txn
room_id (str)
event_list (list)
limit (int)'

def get_backfill_events(self, room_id, event_list, limit):

    return self.runInteraction('get_backfill_events', self._get_backfill_events, room_id, event_list, limit).addCallback(self._get_events).addCallback((lambda l: sorted(l, key=(lambda e: (- e.depth)))))
