1. Department of Computer Science, University of California at Santa Cruz, Santa Cruz, CA 95064, USA.
2. Department of Mechanical and Aeronautical Engineering, University of California at Davis, Davis, CA 95616, USA.
3. Santa Fe Institute, 1399 Hyde Park Road, Santa Fe, NM 87501, USA.
4. Courant Institute of Mathematical Sciences, New York University, New York, NY 10012, USA.