def test_height_of_known_height_tree(self):
   '''
   assert that the height of a tree known to be 4 is actually 4
   '''
   t = Tree()
   t.add(1)
   t.add(2)
   t.add(3)
   t.add(4)
   self.assertEqual(t.height(), 4)
 def test_height_of_zero_node_tree(self):
   '''
   assert that the height of a tree with no nodes should be 0
   '''
   t = Tree()
   self.assertEqual(t.height(), 0)