I thank Aaron Lauda for preparing most of the figures in this paper, and thank Bob Coecke, David Corfield, Issar Stubbe and especially James Dolan for helpful conversations. I would also like to thank the Physics Department of U. C. Santa Barbara for inviting me to speak on this subject, and Fernando Souza for inviting me to speak on this subject at the American Mathematical Society meeting at San Francisco State University, both in October 2000.