ACM, 2012. This is the authors version of the work. It is posted here by permission of ACM for your personal use. Not for redistribution. The definitive version was published in Proceedings of the 44th Symposium on Theory of Computing Conference (STOC'12). ACM, New York, NY, USA, 961-970. http://dx.doi.org/10.1145/2213977.2214064.
Joanne Talbot Hanley
Last modified: Mon Aug 6 15:57:53 EDT 2012