George Metcalfe Nicola Olivetti Dov M. Gabbay Goal-Directed Methods for Lukasiewicz Logic. 85-99 http://springerlink.metapress.com/openurl.asp?genre=article&issn=0302-9743&volume=3210&spage=85 2004 conf/csl/2004 CSL db/conf/csl/csl2004.html#MetcalfeOG04