Gmap.EdgeProvide a mapping function from a mapping of edges.
val map : ( G_Src.E.t -> G_Dst.edge ) -> G_Src.t -> G_Dst.tmap f g applies f to each edge of g and so builds a new graph based on g
val filter_map : ( G_Src.E.t -> G_Dst.edge option ) -> G_Src.t -> G_Dst.tfilter_map f g applies f to each edge of g and so builds a new graph based on g; if None is returned by f the edge is omitted in the new graph.