;; A simple depth-first search example adapted from Winston & Horn

(setf (get 's 'neighbors) '(a d)
      (get 'a 'neighbors) '(s b d)
      (get 'b 'neighbors) '(a c e)
      (get 'c 'neighbors) '(b)
      (get 'd 'neighbors) '(s a e)
      (get 'e 'neighbors) '(b d f)
      (get 'f 'neighbors) '(e))

(defun depth-first (start finish &optional
                       (queue (list (list start))))
     (cond ((endp queue) nil)
	   ((equal finish (first (first queue)))
	       (reverse (first queue)))
	   (t (depth-first start finish
                             (append (extend (first queue))
                                     (rest queue))))))

(defun extend (path)
	(print (reverse path))
	(mapcar #'(lambda (new-node) (cons new-node path))
             (remove-if #'(lambda (neighbor) (member neighbor path))
                        (get (first path) 'neighbors))))