def test_is_bst(self):
bt = BinaryTree(10)
leaf = bt.add_left(7)
bt.add_right(11, leaf)
bt.add_right(39)
self.assertFalse(bt.is_bst())
bt = BinaryTree(10)
leaf = bt.add_left(7)
bt.add_right(8, leaf)
bt.add_right(39)
self.assertTrue(bt.is_bst())
