(define g (graph '(boston nyc dc) '((boston dc) (dc nyc) (boston nyc))))
(describe g)
(new-double-attribute! 'cost 0.0 g)  ; default value is 0.0
(find-double-attribute 'cost (edge-ref g 0))  ; boston-dc
(set-double-attribute! 'cost 132.22 (edge-ref g 1))  ; boston-nyc
(set-double-attribute! 'cost 222.34 (edge-ref g 2))  ; dc-nyc
(find-double-attribute 'cost (edge-ref g 1))  ; boston-nyc
