Institution: | a Department of Mathematics, University of Catania, Citta' universitaria, viale A. Doria 6, 95125 Catania, Italy b Department of Mathematics, University of Torino, via Carlo Alberto 10, 10123 Torino, Italy c Department of Computer Science and Mathematical Logic, Charles University, Malostranské nám. 25, 11800 Praha 1, Czech Republic |