I hadn't carefully read the puzzle conditions of the Kopf und Kopf puzzle mentioned by Thomas Richard, I didn't realize that the digits had to be unique. He asked for a hint on how to handle that with my recursive method. The idea is simple, add a parameter (to the recursive procedure) that supplies the available digits. I've attached the code that does this. This would have been a response, rather than a separate blog entry, however, there does not appear to be a way to add attachments to a response.