diff --git a/src/binary_formats.ml b/src/binary_formats.ml
index 4e37cd05..27a2648a 100644
--- a/src/binary_formats.ml
+++ b/src/binary_formats.ml
@@ -163,7 +163,9 @@ module UTIL_HASH = struct
   end
 
   (* sha512 but add a null termination to the given string
-     this is "HashCode" in GNU TALER *)
+     this is "HashCode" in GNU TALER
+
+     todo: maybe need to have a cstring.ml *)
   module Cstring_hash_64 : H_sig = struct
     include HHH_64