code: def get_transfer_stop_pairs(self):
          
          transfer_stop_pairs = []
          previous_arrival_stop = None
          current_trip_id = None
          for leg in self.legs:
              if leg.trip_id is not None and leg.trip_id != current_trip_id and previous_arrival_stop is not None:
                  transfer_stop_pair = (previous_arrival_stop, leg.departure_stop)
                  transfer_stop_pairs.append(transfer_stop_pair)
              previous_arrival_stop = leg.arrival_stop
              current_trip_id = leg.trip_id
          return transfer_stop_pairs
docs: Get stop pairs through which transfers take place
  
          Returns
          -------
          transfer_stop_pairs: list