1 (defun louis-queens (board-size)
2   (louis-queen-cols board-size board-size))
3 
4 (defun louis-queen-cols (k board-size)
5   (if (= k 0)
6       (list empty-board)
7       (filter (lambda (positions) (safe k positions))
8 	      (flatmap
9 	       (lambda (new-row)
10 		 (mapcar
11 		  (lambda (rest-of-queens)
12 		    (adjoin-position new-row k rest-of-queens))
13 		  (louis-queen-cols (1- k) board-size)))
14 	       (enumerate-interval 1 board-size)))))