def test_double_letter_middle(self):
     s = 'azza'
     self.assertTrue(double_letter(s))
 def test_double_letter_end(self):
     s = 'aczz'
     self.assertTrue(double_letter(s))
 def test_double_letter_beginning(self):
     s = 'aaza'
     self.assertTrue(double_letter(s))