Sackler School of Mathematics and Blavatnik School of Computer Science, Raymond and Beverly Sackler Faculty of Exact Sciences, Tel Aviv University, Tel Aviv, Israel;Department of Computer Science, Technion, Israel;Department of Computer Science, Ben-Gurion University of the Negev, Beer-Sheva, Israel;LIP6, Universite Pierre et Marie Curie, Paris 6, INRIA, France;LIP6, Universite Pierre et Marie Curie, Paris 6, INRIA, France;LIP6, Universite Pierre et Marie Curie, Paris 6, INRIA, France