In preparing this manuscript, I have received assistance from many people, including Michael Bernstein, Steve Bisset, Grant Chang-Chien, Lauren Feiler, Alex Fogel, Ben Golub, George Hines, Richard Jones, Jorge Martínez, Joshua Moses, Dr. John Ryan, and Wei Eileen Xie. I am especially indebted to Anthony B. Williams for a careful, detailed reading of the manuscript yielding hundreds of improvements.