School of Computer Science, University of Manchester, Manchester, M13 9PL, UK. steve.pettifer@manchester.ac.uk|School of Computer Science, University of Manchester, Manchester, M13 9PL, UK. steve.pettifer@manchester.ac.uk|School of Computer Science, University of Manchester, Manchester, M13 9PL, UK. steve.pettifer@manchester.ac.uk|School of Computer Science, University of Manchester, Manchester, M13 9PL, UK. steve.pettifer@manchester.ac.uk|School of Computer Science, University of Manchester, Manchester, M13 9PL, UK. steve.pettifer@manchester.ac.uk|School of Computer Science, University of Manchester, Manchester, M13 9PL, UK. steve.pettifer@manchester.ac.uk|School of Computer Science, University of Manchester, Manchester, M13 9PL, UK. steve.pettifer@manchester.ac.uk