Source: DOI-Technical Report DOI-TR 128, Department of Informatics, Kyushu University, Fukuoka, Japan, December 12, 1996.
* A substantial part of this work has been done while the second author was visiting the Research Institute of Fundamental Information Science (RIFIS) (now Department of Informatics) of Kyushu University at Fukuoka, Japan. This visit has been supported by the Japanese Society for the Promotion of Science under Grant No. 106011. He is greatfully indebted to Setsuo Arikawa for providing excellent working conditions during his stay at RIFIS.
** The fifth author kindly acknowledges the support by the Grant-in-Aid for Scientific Research (C) from the Japan Ministry of Education, Science, Sports, and Culture under Grant No. 07680403.
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