1. Department of Mathematics University of Reading, UK
2. Department of Computer Science Royal Holloway & King's College, University of London, UK. E-mail: J.Daykin@cs.rhul.ac.uk, jackie.daykin@kcl.ac.uk
3. Algorithms Research Group, Department of Computing & Software, McMaster University, Hamilton ON L8S 4K1, Canada and Digital Ecosystems and Business Intelligence Institute Curtin University, GPO Box U1987, Perth WA 6845, Australia. E-mail: smyth@mcmaster.ca