[1] Univ. Cattol. Dipto. di Matemat., Via Tireste 17, 25100 Brescia, Italy
[2] Dept. of Comp. Sci. and Mathematics, University of Stirling, Stirling FK9 4LA, United Kingdom
[3] Department of Computing, Imp. Coll. Sci., Technol. and Med., London, SW7 2AZ, United Kingdom
[4] City University, Northampton Square, EC1 VOH B, United Kingdom