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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, sensor):
 """Add a new sensor to the tree. Parameters sensor : :class:`katcp.Sensor` object New sensor to add to the tree. """

        self._parent_to_children[sensor] = set()
        self._child_to_parents[sensor] = set()