1. Program in Applied and Computational Mathematics, Princeton University, Fine Hall, Princeton, NJ 08544, USA
2. Center for Theoretical Physics, 6-403, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
3. Department of Physics and Program in Applied and Computational Mathematics, Princeton University, Fine Hall, Princeton, NJ 08544, USA
4. Thinking Machines Corporation, 245 First Street, Cambridge, MA 02142, USA