1. Department of Computer Science, University of Sheffield, Sheffield S10 2TN, UK
2. Information School, University of Sheffield, Sheffield S10 2TN, UK
3. School of Computer Science, University of Hull, Hull HU6 7RX, UK
4. Computer Science Department, University of Rome La Sapienza, 00185 Rome, Italy
5. Centre for Regional Economic and Social Research, Sheffield Hallam University, Sheffield S1 1WB, UK