http://reply.papertrans.cn/3/214/21301/21301-11.png
http://reply.papertrans.cn/3/214/21301/21301-12.png
http://reply.papertrans.cn/3/214/21301/21301-13.png
http://reply.papertrans.cn/3/214/21301/21301-14.png
http://reply.papertrans.cn/3/214/21301/21301-15.png
http://reply.papertrans.cn/3/214/21301/21301-16.png
Submitted on: 12 April 2000.
Revised on: 30 April 2000.
Accepted on: 09 May 2000.
NULL
http://reply.papertrans.cn/3/214/21301/21301-18.png
http://reply.papertrans.cn/3/214/21301/21301-19.png
http://reply.papertrans.cn/3/214/21301/21301-20.png