diff --git a/src/tools.ml b/src/tools.ml
index e69de29..57f7f2b 100644
--- a/src/tools.ml
+++ b/src/tools.ml
@@ -0,0 +1,7 @@
+(* Yes, we have to repeat open Graph. *)
+open Graph
+
+(* assert false is of type ∀α.α, so the type-checker is happy. *)
+let clone_nodes gr = assert false
+let gmap gr f = assert false
+let add_arc g id1 id2 =
\ No newline at end of file
diff --git a/src/tools.mli b/src/tools.mli
index e69de29..d2b4af8 100644
--- a/src/tools.mli
+++ b/src/tools.mli
@@ -0,0 +1,5 @@
+open Graph
+
+val clone_nodes: 'a graph -> 'b graph
+val gmap: 'a graph -> ('a -> 'b) -> 'b graph
+val add_arc: int graph -> id -> id -> int -> int graph
\ No newline at end of file