Affiliation: | (1) Centrum voor Wiskunde en Informatica (CWI), P.O. Box 94079, 1090 GB Amsterdam, The Netherlands;(2) Department of Quantitative Economics, Universiteit Maastricht, P.O. Box 616, 6200, MD, Maastricht, The Netherlands;(3) Centre for Discrete Mathematics and its Applications (DIMAP), University of Warwick, Coventry, CV4 7AL, UK;(4) Dipartimento di Informatica e Sistemistica, Università di Roma “La Sapienza”, Via Buonarroti 12, 00185 Roma, Italy |