function for only appending unique items to a list.
    #! consider the possibility of item using this to a set

def unique_append(self, value):
    """ function for only appending unique items to a list.
    #! consider the possibility of item using this to a set
    """
    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