Roberto Sebastiani Paolo Giorgini John Mylopoulos Simple and Minimum-Cost Satisfiability for Goal Models. 20-35 http://springerlink.metapress.com/openurl.asp?genre=article&issn=0302-9743&volume=3084&spage=20 2004 conf/caise/2004 CAiSE db/conf/caise/caise2004.html#SebastianiGM04