code: def unique_append(self, value):
      
      if value not in self:
          try:
              super(self.__class__, self).append(Uri(value))
          except AttributeError as err:
              if isinstance(value, MODULE.rdfclass.RdfClassBase):
                  super(self.__class__, self).append(value)
              else:
                  raise err
docs: function for only appending unique items to a list.
      #! consider the possibility of item using this to a set