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Submitted on: 13 November 2004.
Revised on: 14 February 2005.
Accepted on: 01 March 2005.
___________________HEART---BMJ PUBLISHING GROUP
Submitted on: 06 December 1998.
Revised on: 20 January 1999.
Accepted on: 05 February 1999.
___________________HEART---BMJ PUBLISHING GROUP