(* Yes, we have to repeat open Graph. *)
open Graph

(* assert false is of type ∀α.α, so the type-checker is happy. *)

(* returns a new graph having the same nodes than gr, but no arc. *)
let clone_nodes gr = n_fold gr new_node empty_graph
(*adds an arc or its value if there is an existing one*)
let add_arc g id1 id2 n =
    match find_arc g id1 id2 with
        | None -> new_arc g id1 id2 n
        | Some a -> new_arc g id1 id2 (n + a)

(* maps all arcs of gr by function f *)
let gmap gr f = e_fold gr (fun acc id1 id2 x -> new_arc acc id1 id2 (f x)) (clone_nodes gr)