I should like to express my gratitude to my supervisor, Professor H. Schultink, whose criticisms and careful reading helped me to improve considerably upon the clarity of exposition while I wrote this study, and whose seemingly innocent requests to elaborate confronted me time and again with the need of revising or abandoning ideas I thought stood on firm ground. His support, and Dr. M. C. van den Toom's gratefully acknowl edged willingness to read and evaluate the manuscript enabled me to present this work as a thesis in the University of Utrecht. In more than one way, lowe a debt to Albert...