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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 subscribed_tracks(self):
 """ Access the subscribed_tracks :returns: twilio.rest.video.v1.room.room_participant.room_participant_subscribed_track.SubscribedTrackList :rtype: twilio.rest.video.v1.room.room_participant.room_participant_subscribed_track.SubscribedTrackList """

        if self._subscribed_tracks is None:
            self._subscribed_tracks = SubscribedTrackList(
                self._version,
                room_sid=self._solution['room_sid'],
                subscriber_sid=self._solution['sid'],
            )
        return self._subscribed_tracks