def test_equal_1(self):
     self.assertEqual(find_missing_letter(['a', 'b', 'c', 'd', 'f']), 'e')
 def test_equal_2(self):
     self.assertEqual(find_missing_letter(['O', 'Q', 'R', 'S']), 'P')
 def test_equal_3(self):
     self.assertEqual(find_missing_letter(['b', 'd']), 'c')