1. Department of Mathematics and Computer Science, Bar-Ilan University, Ramat Gan, Israel and Department of Applied Mathematics, The Weizmann Institute of Science, Rehovot, Israel
2. Department of Mathematics and Computer Science, Bar-Ilan University, Ramat Gan, Israel
3. Artificial Intelligence Laboratory, Stanford University, Stanford, California
4. Department of Applied Mathematics, The Weizmann Institute of Science, Rehovot, Israel