def backward_transfer(self, matrix):
        """
        Lower triangular average
        Evaluate learning without forgetting
        """
        r, _ = matrix.shape
        res = [matrix[i, j] for i in range(r) for j in range(i)]
        return sum(res) / len(res)

        Lower triangular average
        Evaluate learning without forgetting
        
avalanche/benchmarks/classic/clear.py