Affiliation: | A1 Department of Computer Science, University of Manchester, Manchester, UK E-mail: horrocks@cs.man.ac.uk A2 Bell Labs Research, Murray Hill, MNJ, USA E-mail: pfps@research.bell-labs.com A3 Università di Trento, Via Inama 5, I-38100, Trento, Italy E-mail: rseba@cs.unitn.it |