<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def stops(self):
 """Return stops served by this route."""

    serves = set()
    for trip in self.trips():
      for stop_time in trip.stop_times():
        serves |= stop_time.stops()
    return serves