def test_is_not_bst(self):
        tree = BSTNode(3)
        tree.left_branch = BSTNode(2)
        tree.right_branch = BSTNode(1)

        self.assertFalse(is_BST(tree))
    def test_is_bst(self):
        tree = BSTNode(2)
        tree.left_branch = BSTNode(1)
        tree.right_branch = BSTNode(3)

        self.assertTrue(is_BST(tree))