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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 stream_messages(self):
 """ Access the stream_messages :returns: twilio.rest.sync.v1.service.sync_stream.stream_message.StreamMessageList :rtype: twilio.rest.sync.v1.service.sync_stream.stream_message.StreamMessageList """

        if self._stream_messages is None:
            self._stream_messages = StreamMessageList(
                self._version,
                service_sid=self._solution['service_sid'],
                stream_sid=self._solution['sid'],
            )
        return self._stream_messages