; 12. Write a function that substitutes an element through another one at all levels of a given list.


; substitute-element (old new lst):
;   new if lst is atom and equal to old
;   lst if lst is atom and not equal to old
;   (substitute-element old new x) for each x in lst otherwise

(defun substitute-element (old new lst)
  (cond ((and (atom lst) (equal lst old)) new)
        ((atom lst) lst)
        (t (mapcar #'(lambda (x) (substitute-element old new x)) lst))))


(print (substitute-element 'a 'z '(a (b a) (c (d a e)))))
(exit)