Department of Computer Science, University of Hong Kong, Hong Kong. whwong@cs.hku.hk|Department of Computer Science, University of Hong Kong, Hong Kong. whwong@cs.hku.hk|Department of Computer Science, University of Hong Kong, Hong Kong. whwong@cs.hku.hk|Department of Computer Science, University of Hong Kong, Hong Kong. whwong@cs.hku.hk|Department of Computer Science, University of Hong Kong, Hong Kong. whwong@cs.hku.hk