docs: public static double[][] getMatrix(final double[][] m1, final int[] r, final int c0, final int c1) {
      assert c0 <= c1 : ERR_INVALID_RANGE;
      assert c1 <= m1[0].length : ERR_MATRIX_DIMENSIONS;
      final int rowdim = r.length, coldim = c1 - c0;
      final double[][] X = new double[rowdim][coldim];
      for(int i = 0; i < rowdim; i++) {
        System.arraycopy(m1[r[i]], c0, X[i], 0, coldim);
      }
      return X;
    }