gistfile1.lisp

(defun a-ikutu (lst)
  (length (filter #'(lambda (x) (eq x 'a)) lst)))


;; filter関数
(defun filter (pred lst)
  (cond ((null lst) nil)
        ((funcall pred (car lst))
         (cons (car ls) (filter pred (cdr lst))))
        (t (filter pred (cdr lst)))))