Test: fix a bug in the dfs-list algo

test/dfs-list/root.ml

-(* Time-stamp: <modified the 19/02/2020 (at 09:56) by Erwan Jahier> *)
+(* Time-stamp: <modified the 25/02/2020 (at 17:20) by Erwan Jahier> *)
 
 (* cf Collin-Dolex-94 *)
 
@@ -14,11 +14,11 @@ let (init_state: int -> 'v) =
 
 let (enable_f: 'v -> 'v neighbor list -> action list) =
   fun v _nl ->
-    if v.path = [] then [] else ["update_path"]
+    if v.path = [-1] then [] else ["update_path"]
 
    
 let (step_f : 'v -> 'v neighbor list -> action -> 'v) =
   fun v nl -> 
     function
-    | "update_path" ->  { v with path = [] }
+    | "update_path" ->  { v with path = [-1] }
     | _ -> assert false