Affiliation: | 1 Center for Logic and Computation, IST, Lisboa 1049-001, Portugal. Email: rchadha{at}math.ist.utl.pt, 2 Dipartimento di Informatica Universitá Ca' Foscari 30172 Venezia, Italy. Email: mace{at}dsi.unive.it, 3 ECS, University of Southampton. Email: vs{at}ecs.soton.ac.uk |