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