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Submitted on: 30 March 2003.
Revised on: 24 April 2003.
Accepted on: 03 June 2003.
___________________JOURNAL OF THE LEARNING SCIENCES
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Submitted on: 07 July 2001.
Revised on: 12 August 2001.
Accepted on: 31 August 2001.
___________________JOURNAL OF THE LEARNING SCIENCES