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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 append(self, cpe):
 """ Adds a CPE element to the set if not already. Only WFN CPE Names are valid, so this function converts the input CPE object of version 2.3 to WFN style. :param CPE cpe: CPE Name to store in set :returns: None :exception: ValueError - invalid version of CPE Name """


        if cpe.VERSION != CPE2_3.VERSION:
            errmsg = "CPE Name version {0} not valid, version 2.3 expected".format(
                cpe.VERSION)
            raise ValueError(errmsg)

        for k in self.K:
            if cpe._str == k._str:
                return None

        if isinstance(cpe, CPE2_3_WFN):
            self.K.append(cpe)
        else:
            # Convert the CPE Name to WFN
            wfn = CPE2_3_WFN(cpe.as_wfn())
            self.K.append(wfn)