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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 clean(self):
 """ Prevents cycles in the tree. """

        super(CTENode, self).clean()
        if self.parent and self.pk in getattr(self.parent, self._cte_node_path):
            raise ValidationError(_("A node cannot be made a descendant of itself."))