Department of Computer Science, Ben Gurion University of the Negev, Beer Sheva 84105, Israel and Microsoft Research Israel, Herzliya 46733, Israel.|Department of Computer Science, Ben Gurion University of the Negev, Beer Sheva 84105, Israel and Microsoft Research Israel, Herzliya 46733, Israel.|Department of Computer Science, Ben Gurion University of the Negev, Beer Sheva 84105, Israel and Microsoft Research Israel, Herzliya 46733, Israel.