docs: private double[] solveLtransposed(double[] X) {
      for(int k = L.length - 1; k >= 0; k--) {
        final double[] Lk = L[k];
        X[k] /= Lk[k];
        for(int i = 0; i < k; i++) {
          X[i] -= X[k] * Lk[i];
        }
      }
      return X;
    }