Department of Computer Science, University of Manchester, Oxford Road, Manchester, M13 9PL, UK. p.lord@russet.org.uk|Department of Computer Science, University of Manchester, Oxford Road, Manchester, M13 9PL, UK. p.lord@russet.org.uk|Department of Computer Science, University of Manchester, Oxford Road, Manchester, M13 9PL, UK. p.lord@russet.org.uk|Department of Computer Science, University of Manchester, Oxford Road, Manchester, M13 9PL, UK. p.lord@russet.org.uk