PhD thesis

• Photocopy of UMCS-91-1-1
• Errata
• When publishing my PhD-thesis as UMCS-91-1-1-1, I was required to keep the number of pages low to save money. This meant small fonts (10 pt), no indentation, narrow page margins, etc., making the thesis unnecessarily hard to read. Over the years, I have also discovered several annoying misprints and minor mistakes. In this revised version, these shortcomings have been removed. Except for the corrections in the bullet above, I have only altered the formatting.