Department of Computer and Information Science, University of Pennsylvania, Philadelphia, Pennsylvania, United States of America. abernal@seas.upenn.edu|Department of Computer and Information Science, University of Pennsylvania, Philadelphia, Pennsylvania, United States of America. abernal@seas.upenn.edu|Department of Computer and Information Science, University of Pennsylvania, Philadelphia, Pennsylvania, United States of America. abernal@seas.upenn.edu|Department of Computer and Information Science, University of Pennsylvania, Philadelphia, Pennsylvania, United States of America. abernal@seas.upenn.edu