Suad Alagic David Briggs Semantics of Objectified XML Constraints. 147-165 http://springerlink.metapress.com/openurl.asp?genre=article&issn=0302-9743&volume=2921&spage=147 2003 conf/dbpl/2003 DBPL db/conf/dbpl/dbpl2003.html#AlagicB03