Acknowledgment

  At the end of an introduction there are usually a lot of thanks. Some remind us of the Genesis, because they go into the minutest details, and of Dan Quayle, because they are just embarrassing. So I am only going to write a few lines to thank:

• Franz Schweiggert for his responsiveness from the very beginning until the end, the huge amount of time which he always had for me, and all his productive tips and hints.
• Matthias Grabert for his help during the testing phase, and his help with dBASE.
• Marie M. Lester for proof reading a huge part of my thesis.
• Christian Plonka for all kinds of support including  perl lessons and checking my English.
• Last, but not least, thanks to my parents who made all this possible.