Dominique Laurent Viet Phan Luong Nicolas Spyratos Querying Weak Instances Under Extension Chase Semantics: A Complete Solution. 591-613 2003 80 Int. J. Comput. Math. 5 http://taylorandfrancis.metapress.com/openurl.asp?genre=article&issn=0020-7160&volume=80&issue=5&spage=591 db/journals/ijcm/ijcm80.html#LaurentLS03