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