1. University Lille, CNRS, Centrale Lille, Inria, UMR 9189-CRIStAL, Lille, France;
2. Department of Computer Science, Université Libre de Bruxelles, 1050 Brussels, Belgium;
3. Parc Scientifique de la Haute Borne, Inria Lille-Nord Europe, 59650 Villeneuve d’Ascq, France;
4. Department of Mathematics, Trier University, 54296 Trier, Germany