1. Hebrew University of Jerusalem, Jerusalem, Israel, and NTT Research, Sunnyvale, CA 94085 USA.
2. Reichman University, Herzliya, Israel 4610101.
3. Computer Science and Applied Mathematics, Weizmann Institute of Science, Rehovot 76100, Israel.
4. Department of Computer Science, Cornell University, Ithaca, NY 14853 USA.
5. Bocconi University, Milan 47010, Italy, and Reichman University, Herzliya 46150, Israel.
6. Computer Science, Bar-Ilan University, Ramat-Gan 5290002, Israel.