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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 cancel(self):
 """ Cancel itself and following NOTs as far as possible. Returns the simplified expression. """

        expr = self
        while True:
            arg = expr.args[0]
            if not isinstance(arg, self.__class__):
                return expr
            expr = arg.args[0]
            if not isinstance(expr, self.__class__):
                return expr