 This entry has benefited from comments, criticisms, and other feedback by Jeremy Avigad, Hourya Benis-Sinaceur, Stanley Burris, William Demopoulos, Diego Fernandes, José Ferreirós, Jeremy Heis, Ansten Klev, Stephan Müller-Stach, Katrin Scheel, Georg Schiemer, Dirk Schlimm, Wilfried Sieg, and the editors. As I have not adopted all of their suggestions, the remaining inaccuracies should all be attributed to me. 