http://reply.papertrans.cn/1/64/6394/6394-31.png
http://reply.papertrans.cn/1/64/6394/6394-32.png
http://reply.papertrans.cn/1/64/6394/6394-33.png
http://reply.papertrans.cn/1/64/6394/6394-34.png
Submitted on: 06 April 2000.
Revised on: 21 July 2000.
Accepted on: 19 August 2000.
___________________CONSTRAINTS---SPRINGER
http://reply.papertrans.cn/1/64/6394/6394-36.png