Submitted on: 10 May 1999.
Revised on: 29 May 1999.
Accepted on: 16 June 1999.
NULL
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Submitted on: 21 July 2000.
Revised on: 17 August 2000.
Accepted on: 01 September 2000.
NULL
Submitted on: 17 May 2008.
Revised on: 05 June 2008.
Accepted on: 14 June 2008.
NULL
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http://reply.papertrans.cn/3/205/20435/20435-36.png