@Test public void set_difference_3() 
{

    Set<Integer> x1 = set(1,2,3) ;
    Set<Integer> x2 = set(4,5,6) ;
    Set<Integer> x3 = SetUtils.difference(x1, x2) ;
    test(x3,1,2,3) ;
    x3 = SetUtils.difference(x2, x1) ;
    test(x3,4,5,6) ;
}