1. CNRS, Université de Nice — Sophia Antipolis, 250 avenue A. Einstein, Bat. 4, Sophia-Antipolis 1, 06560 Valbonne Cedex, France
2. School of Computing Science, Simon Fraser University, Burnaby, British Columbia, V5A IS6, Canada
3. Philips Research Laboratory, Louvain-la-Neuve, Belgium