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