1. School of Computer Science, University of Birmingham, Birmingham B15 2TT, UK
2. Department of Computer Science, Loughborough University, Loughborough LE11 3TT, UK
3. Department of Mathematical Sciences, Loughborough University, Loughborough LE11 3TT, UK
4. Vice-Chancellor’s Office, Keele University, Keele, Newcastle ST5 5BG, UK