def yaml(self):
        return yaml.dump(self.serialized, default_flow_style=False,
                         encoding=None)
def test_supertype(self):
        self.assert_(isinstance(None, NullType))
        self.assert_(isinstance(Optional('a'), NullType))
        self.assert_(isinstance(NotPassed, NotPassedType))
        self.assert_(isinstance(NotPassed, NullType))