First Author:   Yoichiro Fujii Graduate School of Systems and Information Engineering, University of Tsukuba yfujii@sk.tsukuba.ac.jp   1-1-1 Tennoudai Tsukuba, 305-8573, Japan   +810298535547(Phone) +810298535547(Fax) Second Author: Yutaka Nakamura Graduate School of Systems and Information Engineering, University of Tsukuba nakamura@sk.tsukuba.ac.jp   Tsukuba, 305-8573, Japan   +810298535547(Phone) +810298535070(Fax) http://infoshako.sk.tsukuba.ac.jp/~nakamura/index.htm