def result_key(self, k: int) -> Optional[float]:
        """
        Backward Transfer is returned only for keys encountered twice.
        Backward Transfer is the negative forgetting.

        :param k: the key for which returning backward transfer. If k has not
            updated at least twice it returns None.

        :return: the difference between the last value encountered for k
            and its first value.
            It returns None if k has not been updated
            at least twice.
        """

        forgetting = super().result_key(k)
        bwt = forgetting_to_bwt(forgetting)
        return bwt

        Backward Transfer is returned only for keys encountered twice.
        Backward Transfer is the negative forgetting.

        :param k: the key for which returning backward transfer. If k has not
            updated at least twice it returns None.

        :return: the difference between the last value encountered for k
            and its first value.
            It returns None if k has not been updated
            at least twice.
        
avalanche/evaluation/metrics/forgetting_bwt.py