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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 _get_inbound_stream(self, stream_id):
 """ Get or create the inbound stream with the specified ID. """

        if stream_id not in self._inbound_streams:
            self._inbound_streams[stream_id] = InboundStream()
        return self._inbound_streams[stream_id]