let string_of_node_id = function
(id, None) -> string_of_id id
| (id, Some (id2, None)) ->
Printf.sprintf "%s:%s"
(string_of_id id)
(string_of_id id2)
| (id, Some (id2, Some c)) ->
Printf.sprintf "%s:%s:%s"
(string_of_id id)
(string_of_id id2)
(string_of_compass_pt c)