diff --git a/src/tools.ml b/src/tools.ml
index c364da7..7a8836e 100644
--- a/src/tools.ml
+++ b/src/tools.ml
@@ -6,4 +6,9 @@ open Graph
 (* returns a new graph having the same nodes than gr, but no arc. *)
 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);
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+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)
\ No newline at end of file