A short true story: Three friends - two doctors and a mathematician were sitting at a coffee shop. At a moment one of the doctors, say doctor A, said: In medicine we have made great progress in the diagnosis of diseases but less progress has been made in the therapy. Then the other doctor, doctor B, answered: Yes! However this difference between diagnosis and therapy, which shall always exist, is getting smaller and smaller as the years pass. Finally the mathematician spoke: Oh, I see, that what we are talking about, has an analogy to problem solving difficulties. Therapy is, for us, the continuous improvement of problem solving techniques or the invention of new ones. - This is actually what we are trying to do in this book.