Brute force N queens problem solver - SBCL gives up after 7x7 board

queens.lisp

(defun all-pos (n)
  (loop for i from 0 to (- n 1)
    append (loop for j from 0 to (- n 1)
              collect (cons i j))))

(defun single-queen (n)
  (loop for i in (all-pos n)
    collect (list i)))

(defun safep (p moves)
  (every (lambda (m)
           (and (not (= (car p) (car m)))
                (not (= (cdr p) (cdr m)))
                (not (= (abs (- (car p) (car m)))
                        (abs (- (cdr p) (cdr m)))))))
         moves))

(defun merge1 (p all-moves)
  (let ((mergeables (remove-if-not (lambda (ms) (safep p ms)) all-moves)))
    (mapcar (lambda (ms) (cons p ms)) mergeables)))

(defun merge-all (ps all-moves)
  (loop for p in ps
    append (merge1 p all-moves)))

(defun queen1 (n l)
  (let* ((s (single-queen l))
         (p (all-pos l))
         (r s))
    (dotimes (i (- n 1))
      (setq r (merge-all p r)))
    r))