module E:Edges have typeSig.EDGEwith type vertex = vertex
E.t and are labeled with type E.label.
src (resp. dst) returns the origin (resp. the destination) of a
given edge.ORDERED.type t
val compare : t -> t -> inttype vertex
val src : t -> vertexval dst : t -> vertextype label
val create : vertex -> label -> vertex -> tcreate v1 l v2 creates an edge from v1 to v2 with label lval label : t -> label