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Piers Cawley on perl6-internals:
Well, I ported the following Scheme code to PIR. (The PIR is appended to this message...
  ;;; Indicate that the computation has failed, and that the program
  ;;; should try another path.  We rebind this variable as needed.
  (define fail
    (lambda () (error "Program failed")))
  ;;; Choose an arbitrary value and return it, with backtracking.
  ;;; You are not expected to understand this.
  (define (choose . all-choices)
    (let ((old-fail fail))
       (lambda (continuation)
         (define (try choices)
           (if (null? choices)
                 (set! fail old-fail)
                 (set! fail
                      (lambda () (continuation (try (cdr choices)))))
                 (car choices))))
         (try all-choices)))))
  ;;; Find two numbers with a product of 15.
  (let ((x (choose 1 3 5))
        (y (choose 1 5 9)))
    (for-each display `("Trying " ,x " and " ,y #\newline))
    (unless (= (* x y) 15)
    (for-each display `("Found " ,x " * " ,y " = 15" #\newline)))
Which (as anyone can plainly see) implements a non deterministic search (and something like it could come in handy when implementing Perl 6 Junctions).
I was on the floor laughing.