Marc Denecker Nikolay Pelov Maurice Bruynooghe Ultimate Well-Founded and Stable Semantics for Logic Programs with Aggregates. 212-226 2001 conf/iclp/2001 ICLP http://link.springer.de/link/service/series/0558/bibs/2237/22370212.htm db/conf/iclp/iclp2001.html#DeneckerPB01