def test_equals(self): self.assertEqual(substring(''), '')
def test_equals_2(self): self.assertEqual(substring('a'), 'a')
def test_equals_12(self): self.assertEqual(substring('abacddcd'), 'cddcd')
def test_equals_13(self): self.assertEqual(substring('cefageaacceaccacca'), 'accacca')
def test_equals_11(self): self.assertEqual(substring('ccddeeff'), 'ccdd')
def test_equals_10(self): self.assertEqual(substring('bbacc'), 'bba')
def test_equals_9(self): self.assertEqual(substring('abcba'), 'bcb')
def test_equals_8(self): self.assertEqual(substring('abacd'), 'aba')
def test_equals_7(self): self.assertEqual(substring('abc'), 'ab')