And we'll see how important that is as the course goes on. so what about ki, just as a similiar example, what about if we want to count binary trees by external nodes. so that's a different combinatorial class, cuz it's got a different size function, and we denote that with box and T and the atoms are a little bit different, because we consider external nodes to be size one and internal nodes to be a size zero and the generating functions are different. 