Department of Computer Science, University of Manchester, Oxford Road, Manchester M13 9PL, UK. norm@cs.man.ac.uk|Department of Computer Science, University of Manchester, Oxford Road, Manchester M13 9PL, UK. norm@cs.man.ac.uk|Department of Computer Science, University of Manchester, Oxford Road, Manchester M13 9PL, UK. norm@cs.man.ac.uk|Department of Computer Science, University of Manchester, Oxford Road, Manchester M13 9PL, UK. norm@cs.man.ac.uk|Department of Computer Science, University of Manchester, Oxford Road, Manchester M13 9PL, UK. norm@cs.man.ac.uk|Department of Computer Science, University of Manchester, Oxford Road, Manchester M13 9PL, UK. norm@cs.man.ac.uk|Department of Computer Science, University of Manchester, Oxford Road, Manchester M13 9PL, UK. norm@cs.man.ac.uk|Department of Computer Science, University of Manchester, Oxford Road, Manchester M13 9PL, UK. norm@cs.man.ac.uk|Department of Computer Science, University of Manchester, Oxford Road, Manchester M13 9PL, UK. norm@cs.man.ac.uk