bajar-atomos

(defun bajar-atomos (L)
  (labels ((depth (l)
	     (cond ((null l) 0)
		   ((atom (car l)) (depth (cdr l)))
		   (t (max (1+ (depth (car l)))
			   (1+ (depth (cdr l)))))))
	   (rec (l target &optional (depth 1))
	     (cond ((null l) nil)
		   ((atom (car l)) (if (= target depth)
				       nil
				       (cons (car l)
					     (rec (cdr l) target depth))))
		   (t (cons (rec (car l) target (1+ depth))
			    (rec (cdr l) target depth))))))
    (rec L (depth L))))