docs: public static double[][] getMatrix(final double[][] m1, final int[] r, final int[] c) {
      final int rowdim = r.length, coldim = c.length;
      final double[][] X = new double[rowdim][coldim];
      for(int i = 0; i < rowdim; i++) {
        final double[] rowM = m1[r[i]], rowX = X[i];
        for(int j = 0; j < coldim; j++) {
          rowX[j] = rowM[c[j]];
        }
      }
      return X;
    }