(neighbor 'a 'b 7)
(neighbor 'a 'c 9)
(neighbor 'a 'f 14)
(neighbor 'b 'c 10)
(neighbor 'b 'd 15)
(neighbor 'c 'd 11)
(neighbor 'c 'f 2)
(neighbor 'd 'e 6)
(neighbor 'e 'f 9)

(dijkstra 'a 'e)