Institute of Information Science, Academia Sinica, Taipei, 115, Taiwan. arthur@iis.sinica.edu.tw <arthur@iis.sinica.edu.tw>|Institute of Information Science, Academia Sinica, Taipei, 115, Taiwan. arthur@iis.sinica.edu.tw <arthur@iis.sinica.edu.tw>|Institute of Information Science, Academia Sinica, Taipei, 115, Taiwan. arthur@iis.sinica.edu.tw <arthur@iis.sinica.edu.tw>|Institute of Information Science, Academia Sinica, Taipei, 115, Taiwan. arthur@iis.sinica.edu.tw <arthur@iis.sinica.edu.tw>