Department of Computer Science, University of Colorado at Boulder and Howard Hughes Medical Institute, Boulder, CO 80309, USA. antoniog@colorado.edu|Department of Computer Science, University of Colorado at Boulder and Howard Hughes Medical Institute, Boulder, CO 80309, USA. antoniog@colorado.edu|Department of Computer Science, University of Colorado at Boulder and Howard Hughes Medical Institute, Boulder, CO 80309, USA. antoniog@colorado.edu|Department of Computer Science, University of Colorado at Boulder and Howard Hughes Medical Institute, Boulder, CO 80309, USA. antoniog@colorado.edu|Department of Computer Science, University of Colorado at Boulder and Howard Hughes Medical Institute, Boulder, CO 80309, USA. antoniog@colorado.edu