Formated by GeSHi
 
;; Non-deterministic computation
 
(define return-to-the-past
  (let/cc (toplevel)
    (letrec ((top (lambda (ign)
                    (display "; NO MORE OPTIONS!")
                    (newline)
                    (toplevel top))))
      top)))
 
(define-syntax amb*
  (syntax-rules ()
    ((amb* fail)
     (begin
       (fail #f)))
    ((amb* fail first alt ...)
     (let/cc (return)
        (let/cc (cont)
           (set! return-to-the-past cont)
           (return first))
        (amb* fail alt ...)))))
 
(define-syntax amb
  (syntax-rules ()
    ((amb alt ...)
     (let ((fail-continuation return-to-the-past))
       (amb* fail-continuation alt ...)))))
 
(define (require cond)
  (if (not cond)
      (amb)))
 
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