RPN also means that the programming notation is only that. It doesn't give us a data notation, other than a flat sequence of words. This is why Lisp and Forth are not really comparable. In Lisp, you can write made-up words that don't have any previous definition (let alone operator arity), and clump them into whatever nesting you want. As long as you don't try to pass that through the standard evaluator, it's fine.