diff --git a/lib/typeSubstitution.ml b/lib/typeSubstitution.ml
index f35a573..5e2aa4a 100644
--- a/lib/typeSubstitution.ml
+++ b/lib/typeSubstitution.ml
@@ -22,20 +22,25 @@ let rec apply subst = function
   | Type.Arrow (ty1, ty2) -> Type.Arrow (apply subst ty1, apply subst ty2)
 ;;
 
-(** Compose two substitutions *)
+(** Compose two substitutions, last with priority *)
 let compose s2 s1 =
-  IdentifierMap.merge
-    (fun _ ty1 ty2 ->
-      match ty1, ty2 with
-      (* If we have 2, we pick one of them *)
-      | Some ty1', Some _ -> Some (apply s2 ty1')
-      (* If we have 1, we pick the one we have *)
-      | Some ty1', None -> Some (apply s2 ty1')
-      | None, Some ty2' -> Some (apply s2 ty2')
-      (* If we have 0, we return nothing *)
-      | None, None -> None)
-    s1
-    s2
+  let merger =
+    IdentifierMap.merge
+      (* ID type_s1 type_s2 *)
+        (fun _ ty1 ty2 ->
+        match ty1, ty2 with
+        (* Dans ce cas, on donne la priorité à s1 *)
+        | Some ty1', Some _ -> Some ty1'
+        (* Utilisation de la substitution que l'on à déjà *)
+        | Some ty1', None -> Some ty1'
+        | None, Some ty2' -> Some ty2'
+        (* Variable untyped *)
+        | None, None -> None)
+      s1
+      s2
+  in
+  print_endline "wow!";
+  merger
 ;;
 
 let to_string map =