* Department of Computer Science, University of Miami
P.O. Box 248154, Coral Gables, FL 33124, USA
** Department of Theoretical Computer Science and Mathematical Logic,
Charles University
Malostranske namesti 2/25, 118 00 Praha 1, Czech Republic
*** Institut für Informatik 4, Technische Universität München
Boltzmannstr. 3, 85748 Garching b. München, Germany