[math-fun] Earliest Lisp program ?

The discussion of the 40-year anniversary of the Internet in Oct'09 talked about the first message ever sent: "lo" (the first two characters of "login"; the machine crashed before "g"!). I was wondering what was the first non-trivial (i.e., with a loop or recursion) Lisp function ever executed. I would be willing to guess "APPEND", because I think that the first Lisp didn't have numbers, so "FACT" (factorial) wasn't in the running. Does anyone here know the answer?