Submitted on: 04 November 2001.
Revised on: 19 January 2002.
Accepted on: 06 March 2002.
___________________Electronic Markets---SPRINGER HEIDELBERG
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Submitted on: 10 May 2002.
Revised on: 03 September 2002.
Accepted on: 23 September 2002.
___________________Electronic Markets---SPRINGER HEIDELBERG
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