Computer Science Department, Carnegie Mellon University, Pittsburgh PA 15213, USA. tolliver@cs.cmu.edu|Computer Science Department, Carnegie Mellon University, Pittsburgh PA 15213, USA. tolliver@cs.cmu.edu|Computer Science Department, Carnegie Mellon University, Pittsburgh PA 15213, USA. tolliver@cs.cmu.edu|Computer Science Department, Carnegie Mellon University, Pittsburgh PA 15213, USA. tolliver@cs.cmu.edu|Computer Science Department, Carnegie Mellon University, Pittsburgh PA 15213, USA. tolliver@cs.cmu.edu