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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _CreateRouteTripsFolder(self, parent, route, style_id=None, schedule=None):
 """Create a KML Folder containing all the trips in the route. The folder contains a placemark for each of these trips. If there are no trips in the route, no folder is created and None is returned. Args: parent: The parent ElementTree.Element instance. route: The transitfeed.Route instance. style_id: A style id string for the placemarks or None. Returns: The Folder ElementTree.Element instance or None. """

    if not route.trips:
      return None
    trips = list(route.trips)
    trips.sort(key=lambda x: x.trip_id)
    trips_folder = self._CreateFolder(parent, 'Trips', visible=False)
    for trip in trips:
      if (self.date_filter and
          not trip.service_period.IsActiveOn(self.date_filter)):
        continue

      if trip.trip_headsign:
        description = 'Headsign: %s' % trip.trip_headsign
      else:
        description = None

      coordinate_list = []
      for secs, stoptime, tp in trip.GetTimeInterpolatedStops():
        if self.altitude_per_sec > 0:
          coordinate_list.append((stoptime.stop.stop_lon, stoptime.stop.stop_lat,
                                  (secs - 3600 * 4) * self.altitude_per_sec))
        else:
          coordinate_list.append((stoptime.stop.stop_lon,
                                  stoptime.stop.stop_lat))
      placemark = self._CreatePlacemark(trips_folder,
                                        trip.trip_id,
                                        style_id=style_id,
                                        visible=False,
                                        description=description)
      self._CreateLineString(placemark, coordinate_list)
    return trips_folder