This book is the result of a doctoral dissertation written under the super- vision of professor dr. G. de Leve of the University of Amsterdam. I am very grateful to him for suggesting the subject and for his guidance and support during the preparation. Professor dr. ir. J. S. Folkers has carefully read various drafts of the manuscript; I would like to thank him for his many helpful comments and suggestions. I have also greatly benefited from the advice of Gene Lawler, who spent the summer of 1975 in Amsterdam at the invitation of the Stichting Mathematisch Centrum. A quick glance at the...