bajar-atomos

(defun bajar-atomos (L)
  (labels ((flat (l acc)
	     (cond ((null l) acc)
		   ((atom l) (cons l acc))
		   (t (flat (car l)
			    (flat (cdr l) acc)))))
	   (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)
		   ((= target depth) (flat l nil))
		   ((atom (car l)) (cons (car l)
					 (rec (cdr l) target depth)))
		   (t (cons (rec (car l) target (1+ depth))
			    (rec (cdr l) target depth))))))
    (rec L (1- (depth L)))))