;; P04 (*) Find the number of element of a list
(defun my-length (lst)
  (defun my-length-rec (lst res)
    (cond
     ((eq lst '()) res)
     (t
      (my-length-rec (cdr lst) (+ 1 res)))))
  (my-length-rec lst 0))

(my-length '(a b c)) ; 3