Department of Computer Science, University of Western Ontario, London ON, N6A 5B7, Canada.|School of Computing Science, University of Waterloo, Waterloo, ON, N2L 3G1, Canada. lila.kari@uwaterloo.ca.|Department of Mathematics and Computing Science, Saint Marys University, Halifax NS, Canada.|Department of Computer Science, University of Western Ontario, London ON, N6A 5B7, Canada.|Department of Computer Science, University of Western Ontario, London ON, N6A 5B7, Canada.