Agostino Dovier Enrico Pontelli Gianfranco Rossi Intensional Sets in CLP. 284-299 http://springerlink.metapress.com/openurl.asp?genre=article&issn=0302-9743&volume=2916&spage=284 2003 conf/iclp/2003 ICLP db/conf/iclp/iclp2003.html#DovierPR03