;; a helper file for lab9-library.ss
;; Searching a graph.
;;
;;(define-structure (city name nbhrs seen?))


(define (have-seen3? c) (city-seen? c))
(define (mark-seen3! c) (set-city-seen?! c #t))

(define (reachable3? here there)
  (begin
    (mark-seen3! here)
    (if (same-city? here there)
	#t
	(reachable3-from? (city-nbhrs here) there))))


(define (reachable3-from? loc there)
  (cond ((null? loc) #f)
	((if (have-seen3? (car loc))
	     #f
	     (reachable3? (car loc) there))
	 #t)
	(else (reachable3-from? (cdr loc) there))))