78     for (k = M - 1; k >= 0; --k) {
 79         for (i = 0; i < k; ++i)
 80             for (j = 0; j < N; ++j)
 81                 B[i * N + j] = B[i * N + j] - B[k * N + j] * A[i * M + k];
 82     }
 83
 84     for (i = 0; i < M; ++i)
 85         for (j = i; j < N; ++j)
 86             B[i * N + j] *= alpha;