def test_contains(self, size):
        SUBSTR = '0123456789'
        edge = '-' * (size // 2)
        s = ''.join([edge, SUBSTR, edge])
        del edge
        self.failUnless(SUBSTR in s)
        self.failIf(SUBSTR * 2 in s)
        self.failUnless('-' in s)
        self.failIf('a' in s)
        s += 'a'
        self.failUnless('a' in s)