This article is published by Elsevier under a Creative Commons Attribution License and available electronically from http://www.sciencedirect.com/science/article/pii/037026939500935E.
Except where otherwise noted, this item's license is described as: This article is published by Elsevier under a Creative Commons Attribution License and available electronically from http://www.sciencedirect.com/science/article/pii/037026939500935E.