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Submitted on: 23 April 2009.
Revised on: 02 August 2009.
Accepted on: 03 September 2009.
___________________Algebra and Logic---SPRINGER
Submitted on: 22 May 2010.
Revised on: 05 August 2010.
Accepted on: 04 September 2010.
___________________Algebra and Logic---SPRINGER
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