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

        Backward transfer will be returned for all keys encountered at
        least twice.

        :return: A dictionary containing keys whose value has been
            updated at least twice. The associated value is the difference
            between the last and first value recorded for that key.
        """

        forgetting = super().result()
        bwt = forgetting_to_bwt(forgetting)
        return bwt

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

        Backward transfer will be returned for all keys encountered at
        least twice.

        :return: A dictionary containing keys whose value has been
            updated at least twice. The associated value is the difference
            between the last and first value recorded for that key.
        
avalanche/evaluation/metrics/forgetting_bwt.py