Department of Computer Science, Regent Court, 211 Portobello Road, Sheffield S1 4DP, UK. guido@dcs.shef.ac.uk|Department of Computer Science, Regent Court, 211 Portobello Road, Sheffield S1 4DP, UK. guido@dcs.shef.ac.uk|Department of Computer Science, Regent Court, 211 Portobello Road, Sheffield S1 4DP, UK. guido@dcs.shef.ac.uk