Heh, I can explain that. :) I wrote Prof. first but then thought it sounded like I had actually attended his classes, so I removed it. Then, in spanish there is no different wording for PhD vs MD, so we normally only use "doctor" as a title for physicians, or in extremely formal circumstances for other academics. I have always known him as "Andrew Tanenbaum" since the late 80s, so I thought Mr. would be a proper form to show respect.

Thanks for your comment!