 I am indebted to Alan Code, Marc Cohen, and Theodor Ebert for helpful criticisms of earlier versions of this article. I thank Franz Fritsche, Nikolai Biryukov, Ralph E. Kenyon, Johann Dirry, Ben Greenberg, Hasan Masoud, Marc Michael Hämmerling, James Whitely, and edward@logicmuseum.com for calling my attention to errors.