You are now leaving Totseans. Click the link to continue to http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6T4R-3VJS5BP-8&_user=10&_coverDate=12%2F31%2F1998&_rdoc=1&_fmt=high&_orig=search&_origin=search&_sort=d&_docanchor=&view=c&_searchStrId=1647463246&_rerunOrigin=google&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=afe39f394535e29d2c8ef550d614cda5&searchtype=a.