From the first publication of this document, many people have dropped me a note telling me whether they liked it (I love such email!) or telling me about problems they found. Some problems could easily be solved, some others remain a mystery to me. This page enumerates the most important and interesting ones, together with some suggestions from me. This does not mean, though, that I am not interested in reports describing these problems, and I will do my best to help you if you contact me.