1. a Department for Computation and Information , Ruther ord Appleton & Laboratory , Chilton, Oxfordshire, OX110QX, England
2. b Dipartimento di Informatica e Sistemistica , Universitá di Roma "La Sapienza" , Roma, 12-00185, ItalyVia Buonarroti
3. c Department of Mathematics , Facultés Universitaires ND de la Paix , Namur, B-5000, Belgium61 rue de Bruxelles