(define (range n)    (define (recur n)        (if (= n -1)            ‘()            (cons n (recur (- n 1)))))    (recur (- n 1)))(define (flatten a)    (if (null? a)        ‘()        (append (car a) (flatten (cdr a)))))(define (safe? x y sln)    (if (null? sln)        #t        (let ((px (car (car sln))) (py (cadr (car sln))))            (if (or (= y py) (= (- py y) (- px x)) (= (- py y) (- x px)))                #f                 (safe? x y (cdr sln))))))(define (nqueen n)    (define (recur x)        (if (= x -1)            (list ‘())            (flatten (map (lambda (y) (map (lambda (sln) (cons (list x y) sln)) (filter (lambda (sln) (safe? x y sln)) (recur (- x 1))))) (range n)))))    (recur (- n 1)))(define (pl a)    (if (null? a)        ‘()        (begin (display (car a)) (display "\n") (pl (cdr a)))))(pl (nqueen 4))