Submitted on: 28 December 2000.
Revised on: 12 March 2001.
Accepted on: 11 April 2001.
___________________JOURNAL OF PURE AND APPLIED ALGEBRA
Submitted on: 07 September 1998.
Revised on: 26 December 1998.
Accepted on: 06 January 1999.
___________________JOURNAL OF PURE AND APPLIED ALGEBRA
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Submitted on: 12 January 2016.
Revised on: 20 March 2016.
Accepted on: 03 April 2016.
___________________JOURNAL OF PURE AND APPLIED ALGEBRA
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