refactor last tools.ml function

src/tools.ml

@@ -7,3 +7,8 @@ open Graph
 let clone_nodes gr = n_fold gr new_node empty_graph;
 (* maps all arcs of gr by function f *)
 let gmap gr f = e_fold gr (fun acc id1 id2 x -> add_arc id1 id2 (f x)) (clone_nodes gr);
+
+let add_arc g id1 id2 n =
+    match find_arc g id1 id2 with
+        | None -> new_arc(g id1 id2 n graph)
+        | Some a -> new_arc(g id1 id2 n+a graph)