(define (make-big-and-red vert) 
       (set! (color vert) "red")
       (set! (size vert) 20))
(define (big-and-red? vert) 
       (and (equal? (size vert) 20)
            (equal? (color vert) "red")))
(define gv (show-graph (graph '(1 2 3) '((1 2) (2 3)))))
(map big-and-red? (vertices gv))
(map make-big-and-red (vertices gv))
(map big-and-red? (vertices gv))
