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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 add_sensor(self, sensorid=None):
 """Add a sensor to the gateway."""

        if sensorid is None:
            sensorid = self._get_next_id()
        if sensorid is not None and sensorid not in self.sensors:
            self.sensors[sensorid] = Sensor(sensorid)
        return sensorid if sensorid in self.sensors else None