(defun mergesort (li &optional (compare #'>))
  (cond
    ((or (endp li) (endp (rest li))) li)
    (t (labels
	 ((my-split (lis)
		    (cond
		      ((endp lis) (list nil nil))
		      ((endp (rest lis)) (list lis nil))
		      (t (let*
			   ((fi (first lis))
			    (la (first (reverse (rest lis))))
			    (mi (reverse (rest (reverse (rest lis)))))
			    (re (my-split mi)))
			   (list (append (list fi) (first re)) (append (second re) (list la))))))))
	 (let*
	   ((sp (my-split li))
	    (le (mergesort (first sp) compare))
	    (ri (mergesort (second sp) compare)))
	   (merge 'list le ri compare))))))