Department of Computer Science, City University, London EC1V 0HB, UK. drg@cs.city.ac.uk|Department of Computer Science, City University, London EC1V 0HB, UK. drg@cs.city.ac.uk|Department of Computer Science, City University, London EC1V 0HB, UK. drg@cs.city.ac.uk|Department of Computer Science, City University, London EC1V 0HB, UK. drg@cs.city.ac.uk