When writing this book, I was helped in many crucial ways. Thanks must first go to my long-suffering wife, who among other things provided

invaluable help in translating the French edition into English. It is also my delight to acknowledge the help of the following colleagues whose careful reading of the manuscript was expertly done and who suggested numerous improvements on preliminary drafts of this book: Guy Bénat, Jean-Jacques Galzin, Roger Mansuy, Bruno Martin, Landry Salle and Patrick Sargos.