1. Professor in the Department of Electronics and Computer Science, Southampton University, Southampton, SO9 5NH, England.
2. Physics Department, Southampton University, Southampton, SO9 5NH, England.
3. Computer scientist in the Graphics Systems research group at the IBM U.K. Scientific Centre, 2 St. Clements Street, Winchester, SO23 9DR, England.
4. Physics Department, Northeastern University, Boston, MA 02115.