Jörg Fliege
Fachbereich Mathematik
University of Dortmund
44221 Dortmund
Germany
email fliege@math.uni-dortmund.de
Acknowledgments This work was sponsored by the Region Bruxelles Capital under a "Research in Brussels" grant.
I would hereby like to express my gratitude to Prof. Dr. F. Plastria from the Vrije Universiteit Brussel for the kind hospitality, his encouragement, and the many fruitful discussions I enjoyed with him during my stay at the Vakgroep Beleidsinformatica. Moreover, I would like to thank my coworkers and colleagues of the Faculteit van de Economische, Sociale en Politieke Wetenschappen for their help.