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Submitted on: 17 June 1999.
Revised on: 25 July 1999.
Accepted on: 31 August 1999.
___________________BIOMETRIKA---OXFORD UNIV PRESS
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Submitted on: 14 September 2019.
Revised on: 12 October 2019.
Accepted on: 03 November 2019.
___________________BIOMETRIKA---OXFORD UNIV PRESS
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