(eval-when (compile) (localf tak))

(de tak (x y z)
    (cond ((not (ilessp y x))	;x≤y
	   z)
	  (t (tak (tak (isub1 x) y z)
		  (tak (isub1 y) z x)
		  (tak (isub1 z) x y))))) 

(de timit ()
    (do ((n 2 (isub1 n)))
	((izerop n))
	(tak 18 12 6)
	(tak 18 12 6)
	(tak 18 12 6)
	(tak 18 12 6)
	(tak 18 12 6)))