It is a pleasure to acknowledge the help we have received. Charlene Herring, Debbie Herring, Barry Jacobs, and Joseph Miller made their student classroom notes available to us. James Cox, Keith Harrow, Steve Henkind, Karen Lemone, Colm O’Dunlaing, and James Robinett provided helpful comments and corrections. Stewart Weiss was kind enough to redraw one of the figures. Thomas Ostrand, Norman Shulman, Louis Salkind, Ron Sigal, Patricia Teller, and Elia Weixelbaum were particularly generous with their time, and devoted many hours to helping us.