n-queen problem in Common LISP

n-queen.cl

(defun can-place (x y queens)
  (cond 
    ((member t (mapcar #'(lambda (xy) (or (= x (car xy)) (= y (cadr xy)))) queens)) nil)
    ((member t (mapcar #'(lambda (xy) (= 1 (abs (/ (- x (car xy)) (- y (cadr xy)))))) queens)) nil)
    (t)))

; try putting a queen to (x y);
;   if possible, put it and try putting recursively
;   otherwise, trying putting a queen in order: (1 1) ~ (max max)
(defun solve-loop (x y queens max)
  (cond
    ((= max (length queens)) (print (list 'answer queens)) (cdr queens)) ; print answer and backtrack (try to solve more than 1 answer)
    ; ((= max (length queens)) queens)
    ((or (> x max) (> y max)) (cdr queens)) ; backtrack
    ((can-place x y queens)
      (setq rec (solve-loop (+ 1 x) 1 (append (list (list x y)) queens) max))
      (cond
        ((equal queens rec) (solve-loop x (+ 1 y) queens max)) ; backtracked
        (t rec)))
    (t (solve-loop x (+ 1 y) queens max))))

; (trace solve-loop)

(defun solve (max queens)
  (solve-loop 1 1 queens max))

(print (list 'answer (solve 8 '()))) ; try 8 queen