Ton Kloks**, Chuan Min Lee and Jiping Liu**+
**Department of Mathematics and Computer Science
The University of Lethbridge
Alberta, T1K 3M4, Canada
E-mail: {kloks, liu}@cs.uleth.ca
Department of Computer Science and Information Engineering
National Chung Cheng University
Chiayi, 621 Taiwan
E-mail: cmlee@cs.ccu.edu.tw