Affiliation: | 1 Facolta' di Scienze, Universita' di Verona, Strada le Grazie, 37134 Verona, Italy. E-mail: bellin{at}sci.univr.it 2 Department of Computer Science, Queen Mary, University of London, Mile End Road, London E1 4NS, UK. E-mail: kurt{at}dcs.qmul.ac.uk |