code: def append(self, cpe):
          
  
          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)
docs: 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