Bert Bredeweg Paulo Salles Tim Nuttle Using exogenous quantities in qualitative models about environmental sustainability. 49-58 2007 20 AI Commun. 1 http://iospress.metapress.com/openurl.asp?genre=article&issn=0921-7126&volume=20&issue=1&spage=49 db/journals/aicom/aicom20.html#BredewegSN07