def test_string1_empty(self): self.assertEqual('', longest_common_substring('string1', ''))
def test_string1_string2(self): self.assertEqual('string', longest_common_substring('string1', 'string2'))
def test_animal_mallard(self): self.assertEqual('mal', longest_common_substring('animal', 'mallard'))
def test_empty_string2(self): self.assertEqual('', longest_common_substring('', 'string2'))