towers of hanoi | Takeuchi function | combinations of a set | letrec macro
; (comb n a) = N-combinations of set A
; (comb n a t) = with repetition
(defun comb (n a . r)
(labels
((comb (n a)
(cond
((zerop n) nil)
((eqv 1 n) (mapcar list a))
(t (mapcan
(lambda (tl)
(mapcar
(lambda (u)
(cons (car tl) u))
(comb (sub1 n)
(if r tl (cdr tl)))))
(maplist (lambda (x) x) a))))))
(comb n a)))