Department of Computer Science, Brown University, Toyota Technological Institute at Chicago, Chicago, IL, USA. aritz@cs.brown.edu|Department of Computer Science, Brown University, Toyota Technological Institute at Chicago, Chicago, IL, USA. aritz@cs.brown.edu|Department of Computer Science, Brown University, Toyota Technological Institute at Chicago, Chicago, IL, USA. aritz@cs.brown.edu|Department of Computer Science, Brown University, Toyota Technological Institute at Chicago, Chicago, IL, USA. aritz@cs.brown.edu