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(http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V2W-4NS36HK-6&_user=10&_coverDate=09%2F30%2F2007&_rdoc=1&_fmt=high&_orig=search&_origin=search&_sort=d&_docanchor=&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=aa00443f7d5c65c797ca173e20b70a2c&searchtype=a)