diff --git a/a.output b/a.output
new file mode 100644
index 00000000..53eb1eba
Binary files /dev/null and b/a.output differ
diff --git a/b.output b/b.output
new file mode 100644
index 00000000..668632f2
Binary files /dev/null and b/b.output differ
diff --git a/src/binary_formats.ml b/src/binary_formats.ml
index f51dc198..181a9b4f 100644
--- a/src/binary_formats.ml
+++ b/src/binary_formats.ml
@@ -451,15 +451,29 @@ module WithdrawConfirmationPS = struct
     noreveal_index: int32;
   }
 
-  let size = Purpose.size + HashPlanchetsP.size + int32_size
-  let purpose = Purpose.make ~size Taler_signatures.exchange_confirm_withdraw
+  (*let size = Purpose.size + HashPlanchetsP.size + int32_size*)
 
-  let bin =
-    let open Bin in
-    record (fun _urpose h_planchets noreveal_index ->
-        { h_planchets; noreveal_index })
-    |+ field Purpose.bin (Fun.const purpose)
-    |+ field HashPlanchetsP.bin (fun t -> t.h_planchets)
-    |+ field beint32 (fun t -> t.noreveal_index)
-    |> sealr
+  open Bin
+
+  let rec bin =
+    lazy
+      (record (fun _purpose h_planchets noreveal_index ->
+           { h_planchets; noreveal_index })
+      |+ field Purpose.bin (fun _t -> Lazy.force purpose)
+      |+ field HashPlanchetsP.bin (fun t -> t.h_planchets)
+      |+ field beint32 (fun t -> t.noreveal_index)
+      |> sealr)
+
+  and size =
+    lazy
+      (let size_t = Size.of_value (Size.size_of (Lazy.force bin)) in
+       match size_t with
+       | Static n -> n
+       | Dynamic _ | Unknown ->
+           Fmt.failwith "size_of failure: size is not Static")
+
+  and purpose =
+    lazy
+      (let size = Lazy.force size in
+       Purpose.make ~size Taler_signatures.exchange_confirm_withdraw)
 end
diff --git a/test/test.ml b/test/test.ml
index 5629d120..367af4b6 100644
--- a/test/test.ml
+++ b/test/test.ml
@@ -76,12 +76,12 @@ let () =
   let open Binary_formats.WithdrawConfirmationPS in
   let str64 = String.init 64 (fun i -> Char.unsafe_chr (i + 1)) in
   let dummy_t = { h_planchets= { v= { v= str64 } }; noreveal_index= 0_l } in
+  let bin = Lazy.force bin in
+  let size = Lazy.force size in
   let size' = Bin.size_of_value bin dummy_t |> Option.get in
   assert (size = size');
   assert (size = 76);
 
-  (* for cmp test with signatures.c output
+  (* for cmp test with signatures.c output *)
   let raw_str = Bin.to_string bin dummy_t in
-  Printf.printf "%s" raw_str;
-   *)
-  ()
+  Printf.printf "%s" raw_str; ()