docs: public static double[][] getMatrix(final double[][] m1, final int r0, final int r1, final int[] c) {
      assert r0 <= r1 : ERR_INVALID_RANGE;
      assert r1 <= m1.length : ERR_MATRIX_DIMENSIONS;
      final int rowdim = r1 - r0, coldim = c.length;
      final double[][] X = new double[rowdim][coldim];
      for(int i = r0; i < r1; i++) {
        final double[] row = m1[i];
        final double[] Xi = X[i - r0];
        for(int j = 0; j < coldim; j++) {
          Xi[j] = row[c[j]];
        }
      }
      return X;
    }