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Submitted on: 31 July 2011.
Revised on: 21 August 2011.
Accepted on: 06 September 2011.
___________________Annals of Applied Statistics---INST MATHEMATICAL STATISTICS-IMS
Submitted on: 03 January 2021.
Revised on: 02 May 2021.
Accepted on: 05 June 2021.
___________________Annals of Applied Statistics---INST MATHEMATICAL STATISTICS-IMS