docs: public static double[][] getMatrix(final double[][] m1, final int r0, final int r1, final int c0, final int c1) {
      assert r0 <= r1 && c0 <= c1 : ERR_INVALID_RANGE;
      assert r1 <= m1.length && c1 <= m1[0].length : ERR_MATRIX_DIMENSIONS;
      final int rowdim = r1 - r0, coldim = c1 - c0;
      final double[][] X = new double[rowdim][coldim];
      for(int i = r0; i < r1; i++) {
        System.arraycopy(m1[i], c0, X[i - r0], 0, coldim);
      }
      return X;
    }