(labels ((mapatoms (function tree)
           (cond
             ((atom tree)
              (funcall function tree))
             ((consp tree)
              (cons (mapatoms function (car tree))
                    (mapatoms function (cdr tree)))))))
  (mapatoms (lambda (atom)
              (format t "~A~%" atom))
            '(defmacro (x) `(list ,x))))
;; output:
;; DEFMACRO
;; X
;; NIL
;; QUASIQUOTE
;; LIST
;; ,X
;; NIL
;; NIL
;; NIL
;; (NIL (NIL) (NIL (NIL NIL)))
...