def __repr__(self):
        return "<Requirement(package={package}, version='{version}')>".format(
            package=self.package, version=self.version)
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))