diff --git a/bin/day2405.ml b/bin/day2405.ml
index a65cb2b..d277594 100644
--- a/bin/day2405.ml
+++ b/bin/day2405.ml
@@ -1,21 +1,16 @@
 module IntSet = Set.Make (Int)
-module IntHashtbl = Hashtbl.Make (Int)
 
 (** [add_rule a b m] adds the rule that [a] must appear before [b] to the rule
     map [m]. Returns the updated map [m] *)
 let add_rule a b m =
-  match IntHashtbl.find_opt m a with
-  | None ->
-      IntHashtbl.add m a (IntSet.singleton b);
-      m
-  | Some s ->
-      IntHashtbl.replace m a (IntSet.add b s);
-      m
+  match Hashtbl.find_opt m a with
+  | None -> Hashtbl.add m a (IntSet.singleton b)
+  | Some s -> Hashtbl.replace m a (IntSet.add b s)
 
 (** [find_rule a b m] returns [true] if the rule map [m] says that [a] should
     appear before [b]. *)
 let find_rule a b m =
-  match IntHashtbl.find_opt m a with
+  match Hashtbl.find_opt m a with
   | Some s -> ( match IntSet.find_opt b s with Some _ -> true | None -> false)
   | None -> false
 
@@ -45,15 +40,18 @@ let rec is_page_order_valid m pages =
     [tail] starts the line after the empty line. *)
 let parse_rules =
   let re = Str.regexp_string "|" in
-  let rec impl acc = function
-    | "" :: t -> (acc, t)
+  let m = Hashtbl.create 17 in
+  let rec impl = function
+    | "" :: t -> (m, t)
     | [] -> failwith "parse_rules.impl"
     | h :: t -> (
         match List.map int_of_string (Str.split re h) with
-        | [ a; b ] -> impl (add_rule a b acc) t
+        | [ a; b ] ->
+            add_rule a b m;
+            impl t
         | _ -> failwith "parse_rules.impl")
   in
-  impl (IntHashtbl.create 17)
+  impl
 
 (** [parse_page_orders lst] parses a list of page orders.  *)
 let parse_page_orders =