http://t3x.org/s9fes/zebra.scm.html

# Zebra puzzle (using AMK)

Location: contrib, 38 Lines

```; Scheme 9 from Empty Space, Function Library
; By Nils M Holm, 2009
; Placed in the Public Domain
;
; (zebra)  ==>  list
;
; Solve the zebra puzzle using AMK.
;
; Example:   (zebra)  ==>  (((norwegian kools _.0 fox yellow)
;                            (ukrainian chesterfields tea horse blue)
;                            (englishman oldgolds milk snails red)
;                            (spaniard luckystrikes orangejuice dog ivory)
;                            (japanese parliaments coffee zebra green)))

(define (lefto x y l)
(fresh (t)
(cdro l t)
(any (all (caro l x)
(caro t y))
(lefto x y t))))

(define (nexto x y l)
(any (lefto x y l)
(lefto y x l)))

(define (zebra)
(run* (h)
(== h (list (list 'norwegian (_) (_) (_) (_))
(_)
(list (_) (_) 'milk (_) (_))
(_)
(_)))
(memo (list 'englishman (_) (_) (_) 'red) h)
(lefto (list (_) (_) (_) (_) 'ivory)
(list (_) (_) (_) (_) 'green) h)
(nexto (list 'norwegian (_) (_) (_) (_))
(list (_) (_) (_) (_) 'blue) h)
(memo (list (_) 'kools (_) (_) 'yellow) h)
(memo (list 'spaniard (_) (_) 'dog (_)) h)
(memo (list (_) (_) 'coffee (_) 'green) h)
(memo (list 'ukrainian (_) 'tea (_) (_)) h)
(memo (list (_) 'luckystrikes 'orangejuice (_) (_)) h)
(memo (list 'japanese 'parliaments (_) (_) (_)) h)
(memo (list (_) 'oldgolds (_) 'snails (_)) h)
(nexto (list (_) (_) (_) 'horse (_))
(list (_) 'kools (_) (_) (_)) h)
(nexto (list (_) (_) (_) 'fox (_))
(list (_) 'chesterfields (_) (_) (_)) h)
;  (memo (list (_) (_) 'water (_) (_)) h)
(memo (list (_) (_) (_) 'zebra (_)) h)))
```