extern void edge_add(edge_t *e);
extern void edge_del(edge_t *e);
extern edge_t *lookup_edge(struct node_t *from, struct node_t *to);
extern void edge_add(edge_t *e);
extern void edge_del(edge_t *e);
extern edge_t *lookup_edge(struct node_t *from, struct node_t *to);