Affiliation: | 1 Università di Pisa, Dipartimento di Informatica, Largo Bruno Pontecorvo, 3, 56127 Pisa, Italy. E-mail: dipierro{at}di.unipi.it, 2 Department of Computing, Imperial College, 180 Queen's Gate, London SW7 2AZ, UK. E-mail: clh{at}doc.ic.ac.uk, herbert{at}doc.ic.ac.uk |