1. Institute of Mathematics, University of Vienna, Augasse 2-6, UZA 1 - Building 2, 1090 Vienna, Austria
2. Institut für Mathematische Logik und Grundlagenforschung, Universität Münster, Einsteinstr. 62, 48149 Münster, FRG, Germany
3. Department of Mathematics and Department of Philosophy, Harvard University, 2 Arrow Street Room 417, Cambridge, MA 02138, USA