public UiTopoLayout region(Region region) {
        if (isRoot()) {
            throw new IllegalArgumentException(E_ROOT_REGION);
        }

        this.region = region;
        return this;
    }
@Test
    public void setRegionOnRoot() {
        mkRootLayout();
        try {
            layout.region(REGION);
            fail(AM_NOEX);
        } catch (IllegalArgumentException e) {
            assertEquals(AM_WREXMSG, E_ROOT_REGION, e.getMessage());
        }

        try {
            layout.region(null);
            fail(AM_NOEX);
        } catch (IllegalArgumentException e) {
            assertEquals(AM_WREXMSG, E_ROOT_REGION, e.getMessage());
        }
    }