Robert Demolombe An Extended Relational Algebra on Abstract Objects for Summarizing Answers to Queries. 1-15 2003 57 Fundam. Inform. 1 http://iospress.metapress.com/openurl.asp?genre=article&issn=0169-2968&volume=57&issue=1&spage=1 db/journals/fuin/fuin57.html#Demolombe03