Department of Computer Science, University of Western Ontario, N6A 5B7 London, Ontario, Canada. ilie@csd.uwo.ca.|Department of Computer Science, University of Western Ontario, N6A 5B7 London, Ontario, Canada. ilie@csd.uwo.ca.|Department of Computer Science, University of Western Ontario, N6A 5B7 London, Ontario, Canada. ilie@csd.uwo.ca.|Department of Computer Science, University of Western Ontario, N6A 5B7 London, Ontario, Canada. ilie@csd.uwo.ca.