Submitted on: 11 August 2012.
Revised on: 20 September 2012.
Accepted on: 14 November 2012.
___________________JOURNAL OF THE ACM---ASSOC COMPUTING MACHINERY
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Submitted on: 16 November 2001.
Revised on: 18 December 2001.
Accepted on: 03 January 2002.
___________________JOURNAL OF THE ACM---ASSOC COMPUTING MACHINERY
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