1. Department of Mathematics, University of California, Berkeley, California 94720-3840, USA
2. Department of Philosophy, University of Washington, Seattle, Washington 98195-3350, USA
3. Department of Mathematics, Western Washington University, Bellingham, Washington 98225, USA
4. Department of Mathematics, University of Washington, Seattle, Washington 98195-4350, USA
5. Department of Computer Science, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada