1 (defun make-position (row col)
2   (cons row col))
3 (defun position-row (pos)
4   (car pos))
5 (defun position-col (pos)
6   (cdr pos))
7 (defun positions-equal (a b)
8   (equal a b))
9 (defvar empty-board '())
10 (defun adjoin-position (row col positions)
11   (append positions (list (make-position row col))))
12 (defun attacks? (a b)
13   (let ((a-row (position-row a))
14 	(a-col (position-col a))
15 	(b-row (position-row b))
16 	(b-col (position-col b)))
17     (cond
18       ((= a-row b-row) t)
19       ((= a-col b-col) t)
20       ((= (abs (- a-col b-col))
21 	  (abs (- a-row b-row))) t)
22       (t nil))))
23 (defun safe? (k positions)
24   (let ((kth-pos (nth (1- k) positions)))
25     (if (null (find-if
26 	       (lambda (pos)
27 		 (and (not (positions-equal kth-pos pos))
28 		      (attacks? kth-pos pos)))
29 	       positions))
30 	t
31 	nil)))