(set addup (function (L) (if (= L null) 0 (+ (head L) (addup (tail L))))))
(set between (function (A B) (if (> A B) null (cons A (between (+ 1 A) B)))))
(set append (function (A B) (if (= A NULL) B (cons (head A) (append (tail A) B)))))
(set append3 (function (A B C) (append A (append B C))))