1. Faculty of Computer Science, "Alexandru Ioan Cuza" University, 11 Carol I Bd., Iasi, 6600, Romania
2. CentER and Department of Econometrics and Operations Research, Tilburg Universiy, P.O. Box 90153, Tilburg, 5000 LE, The Netherlands
3. Department of Mathematics, Center for Applied Computer Science (ZAIK), University of Cologne, Weyertal 80, Cologne, D-50931, Germany