Hello Anton, thanks for your enlightening piece of code. "foldD" does exactly what I was hoping for. Now it becomes very obvious to me that the positions of the forall's in the Digit definition are crucial. Thanks a lot to Alfonso, Luke and Ryan as well. Haskell Cafe is a really good place to learn new things :-) Harald. " Ce courriel et les documents qui y sont attaches peuvent contenir des informations confidentielles. Si vous n'etes pas le destinataire escompte, merci d'en informer l'expediteur immediatement et de detruire ce courriel ainsi que tous les documents attaches de votre systeme informatique. Toute divulgation, distribution ou copie du present courriel et des documents attaches sans autorisation prealable de son emetteur est interdite." " This e-mail and any attached documents may contain confidential or proprietary information. If you are not the intended recipient, please advise the sender immediately and delete this e-mail and all attached documents from your computer system. Any unauthorised disclosure, distribution or copying hereof is prohibited."