def METHOD_NAME():
    grid = RasterModelGrid((3, 4), xy_spacing=3.0)
    z = [[0.0, 0.0, 0.0, 0.0], [0.0, 5.0, 5.0, 0.0], [0.0, 0.0, 0.0, 0.0]]
    assert_array_equal(
        grid.calc_diff_at_diagonal(z),
        np.array(
            [[5.0, 0.0, 5.0, 5.0, 0.0, 5.0], [0.0, -5.0, -5.0, -5.0, -5.0, 0.0]]
        ).flat,
    )