Code for http://stackoverflow.com/q/20393589/1281433

;; http://stackoverflow.com/q/20393589/1281433
;; Joshua Taylor, 4 December 2013

(defun length1 (list)
  (if (endp list)
      0
      (+ 1 (length1 (rest list)))))

(defun length2 (list)
  (labels ((l2 (list len)
             (if (endp list)
                 len
                 (l2 (rest list) (1+ len)))))
    (l2 list 0)))

(defun length3 (list)
  (do ((list list (rest list))
       (len 0 (1+ len)))
      ((endp list) len)))

(defun ack2 (x y)
  (do () ((zerop x) (1+ y))
    (if (zerop y)
        (setf x (1- x)
              y 1)
        (psetf x (1- x)
               y (ack2 x (1- y))))))

(defun ack3 (x y)
  (do ((x x (1- x))
       (y y (if (zerop y) 1 (ack3 x (1- y)))))
      ((zerop x) (1+ y))))