'A diff composed of two separate ``set`` operations can be applied to an
object without triggering an invariant exception.'

def test_straight_swap(self):

    o1 = DiffTestObjInvariant(a=1, b=2)
    o2 = DiffTestObjInvariant(a=2, b=1)
    diff = create_diff(o1, o2)
    self.expectThat(len(diff.changes), Equals(2))
    self.assertEqual(o2, diff.apply(o1))
