I certainly used Lisp notation, but if you think it's "just Lisp" then you're kind of missing the point. It's not just "just Lisp", it's "just lambda". There is nothing else in there, no primitives, no numbers, no CAR, CDR, CONS, COND or QUOTE. And, to drive home the point that the Y combinator is in there, no DEFUN, LABELS or LETREC. (Well, OK, there's an IF in there, but even that could be gotten rid of with a lazy lambda.)