When Allen T. Craig died in late November 1978, I lost my advisor, mentor, colleague, and very dear friend. Due to his health, Allen did nothing on the fourth edition and, of course, this revision is mine alone. There is, however, a great deal of Craig's influence in this book. As a matter of fact, when I would debate with myself whether or not to change something, I could hear Allen saying, "It's very good now, Bob; don't mess it up." Often, I would follow that advice