f node s n xs f addchild select xs node n xs select cons ap xs ap select cons ap xs select xs addchild node y ys node n xs node y cons node n xs ys 2.2 f node s n xs f addchild select xs node n xs f node s n xs addchild select xs node n xs select cons ap xs select xs f node s n xs select xs true f node s n xs f addchild select xs node n xs true Failed! NaTT certifiable-1.6