1. Cheriton School of Computer Science, University of Waterloo, Canada
2. Univ. Lille, Inria, CNRS, Centrale Lille, UMR 9189 CRIStAL, France and Cheriton School of Computer Science, University of Waterloo, Canada
3. Univ. Lille, CNRS, Inria, Centrale Lille, UMR 9189 CRIStAL, France and Cheriton School of Computer Science, University of Waterloo, Canada