School of Computer Science, University of Manchester, Oxford Road, Manchester M13 9PL, UK. garwood@cs.man.ac.uk <garwood@cs.man.ac.uk>|School of Computer Science, University of Manchester, Oxford Road, Manchester M13 9PL, UK. garwood@cs.man.ac.uk <garwood@cs.man.ac.uk>|School of Computer Science, University of Manchester, Oxford Road, Manchester M13 9PL, UK. garwood@cs.man.ac.uk <garwood@cs.man.ac.uk>|School of Computer Science, University of Manchester, Oxford Road, Manchester M13 9PL, UK. garwood@cs.man.ac.uk <garwood@cs.man.ac.uk>|School of Computer Science, University of Manchester, Oxford Road, Manchester M13 9PL, UK. garwood@cs.man.ac.uk <garwood@cs.man.ac.uk>|School of Computer Science, University of Manchester, Oxford Road, Manchester M13 9PL, UK. garwood@cs.man.ac.uk <garwood@cs.man.ac.uk>|School of Computer Science, University of Manchester, Oxford Road, Manchester M13 9PL, UK. garwood@cs.man.ac.uk <garwood@cs.man.ac.uk>