1. Computer Science and Artificial Intelligence Laboratory, Massachusetts Institute of Technology, Cambridge, MA 02139
2. Ragon Institute of MGH, MIT and Harvard, Cambridge, MA 02139
3. Department of Biological Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139
4. Department of Computer Science, Tufts University, Medford, MA 02155
5. Department of Mathematics, Massachusetts Institute of Technology, Cambridge, MA 02139