一般のNに対してN-queen問題を解くやつ

Nqueen.scm

; (N-queen 7 '((2 3) (4 6))) => ((7 7) (6 2) (5 4) (3 1) (1 5) (2 3) (4 6))
(define (N-queen N ini-points)
  (define (delete-l lst target)
    (if (null? lst) target
	(delete-l (cdr lst) (delete (car lst) target))))
  (define (ok? x y his)
    (let loop ((his his))
      (cond ((null? his) #t)
	    ((= (abs (- x (caar his)))
		(abs (- y (cadar his))))
	     #f)
	    (else (loop (cdr his))))))
  (call/cc
    (^(return)
      (let loop ((xs (delete-l (map car ini-points) (iota N 1)))
		 (ys (delete-l (map cadr ini-points) (iota N 1)))
		 (result ini-points))
	(if (null? xs) (return result)
	    (dolist (y ys)
	      (when (ok? (car xs) y result)
		(loop (cdr xs) (delete y ys) (cons `(,(car xs) ,y) result)))))))))