Department of Computer Science, The University of Hong Kong, Pokfulam, Hong Kong, PRChina. chin@cs.hku.hk|Department of Computer Science, The University of Hong Kong, Pokfulam, Hong Kong, PRChina. chin@cs.hku.hk|Department of Computer Science, The University of Hong Kong, Pokfulam, Hong Kong, PRChina. chin@cs.hku.hk|Department of Computer Science, The University of Hong Kong, Pokfulam, Hong Kong, PRChina. chin@cs.hku.hk