@Test public void set_symmetric_differnce_4() {

    Set<Integer> x1 = set(1,2,3) ;
    Set<Integer> x2 = set() ;
    Set<Integer> expected = set(1,2,3) ;
    assertEquals(expected, SetUtils.symmetricDifference(x1, x2)) ;
    assertEquals(expected, SetUtils.symmetricDifference(x2, x1)) ;
}