def test_difference(c, d, ans):
    from sets.verified_sets import IntSet
    a = IntSet(c)
    b = IntSet(d)
    t = a.difference(b)
    assert (str(t) == ans and isinstance(t, IntSet)), \
        f"Expected difference {ans} and type IntSet"
def test_difference_subclass():
    from sets.verified_sets import IntSet
    a = IntSet(())
    with pytest.raises(TypeError):
        a.difference(("a", "b"))