This book is the product of research which I undertook for my doc- toral thesis. The project was started whilst I was at the Free Univer- sity of Amsterdam, and the State University of Groningen gave me ample opportunity to complete the work. At both universities I was lucky enough to find kind colleagues who were willing to perform my teaching tasks, enabling me to spend much of my time some of on my research. I should like to thank Wietze Boomsma, Kees van den Hoeven and Jan Oosterhaven for their kind help. I was also most encouraged to discover several students at both institutions who...