N Queens

    ;;; NQUEENS -- Compute number of solutions to 8-queens problem.

;; http://wiki.call-cc.org/chicken-compilation-process

    (define (nqueens n)

    (define (dec-to n)
        (let loop ((i n) (l '()))
        (if (= i 0) l (loop (- i 1) (cons i l)))))

    (define (try x y z)
        (if (null? x)
        (if (null? y)
            1
            0)
        (+ (if (ok? (car x) 1 z)
            (try (append (cdr x) y) '() (cons (car x) z))
            0)
            (try (cdr x) (cons (car x) y) z))))

    (define (ok? row dist placed)
        (if (null? placed)
        #t
        (and (not (= (car placed) (+ row dist)))
            (not (= (car placed) (- row dist)))
            (ok? row (+ dist 1) (cdr placed)))))

    (try (dec-to n) '() '()))

    (nqueens 8)