1. Dipartimento di Informatica, Università di Pisa, Via F. Buonarroti 2, I-56127 Pisa, Italy. E-mail: chiara@di.unipi.it, degano@di.unipi.it
2. Informatics and Mathematical Modelling, Technical University of Denmark, Richard Petersens Plads bldg 321, DK-2800 Kongens Lyngby, Denmark. E-mail: mib@imm.dtu.dk, nielson@imm.dtu.dk, riis@imm.dtu.dk