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Submitted on: 29 March 2007.
Revised on: 19 May 2007.
Accepted on: 22 June 2007.
___________________Ingenieria Solidaria---UNIV COOPERATIVE COLOMBIA, FAC ENGINEERING
Submitted on: 09 December 1999.
Revised on: 13 March 2000.
Accepted on: 31 March 2000.
___________________Ingenieria Solidaria---UNIV COOPERATIVE COLOMBIA, FAC ENGINEERING
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