'The stream token for an event
Args:
event_id(str): The id of the event to look up a stream token for.
Raises:
StoreError if the event wasn\'t in the database.
Returns:
A deferred "s%d" stream token.'

def get_stream_token_for_event(self, event_id):

    return self._simple_select_one_onecol(table='events', keyvalues={'event_id': event_id}, retcol='stream_ordering').addCallback((lambda row: ('s%d' % (row,))))
