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Submitted on: 04 December 2004.
Revised on: 04 January 2005.
Accepted on: 18 February 2005.
___________________DIFFERENTIATION---ELSEVIER SCI LTD
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Submitted on: 27 May 2000.
Revised on: 10 July 2000.
Accepted on: 27 July 2000.
___________________DIFFERENTIATION---ELSEVIER SCI LTD
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