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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 pushdown_not(self):
 """Return an expression with NOT operators pushed down thru dual ops. Specifically, perform the following transformations: ~(s ? d1 : d0) <=> s ? ~d1 : ~d0 """

        node = self.node.pushdown_not()
        if node is self.node:
            return self
        else:
            return _expr(node)