Thierry Boy de la Tour Mnacho Echenim NP-Completeness Results for Deductive Problems on Stratified Terms. 317-331 http://springerlink.metapress.com/openurl.asp?genre=article&issn=0302-9743&volume=2850&spage=317 2003 conf/lpar/2003 LPAR db/conf/lpar/lpar2003.html#TourE03