def test_maxVal(self):
        """
        L{SerialNumber.__add__} returns a wrapped value when s1 plus the s2
        would result in a value greater than the C{maxVal}.
        """
        s = SerialNumber(1)
        maxVal = s._halfRing + s._halfRing - 1
        maxValPlus1 = maxVal + 1
        self.assertTrue(SerialNumber(maxValPlus1) > SerialNumber(maxVal))
        self.assertEqual(SerialNumber(maxValPlus1), SerialNumber(0))

        L{SerialNumber.__add__} returns a wrapped value when s1 plus the s2
        would result in a value greater than the C{maxVal}.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/names/test/test_rfc1982.py