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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 set_model(self, model):
 """Set the model the item belongs to A TreeItem can only belong to one model. :param model: the model the item belongs to :type model: :class:`Treemodel` :returns: None :rtype: None :raises: None """

        self._model = model
        for c in self.childItems:
            c.set_model(model)