A pre-relational databases datamodel. "Preceeded" by the relational model since the flexibility of this makes it hard to work with. Now re-invented in RDF :)
G. Tummarello, C. Morbidoni, and M. Nucci. Proceedings of the Fifth International Semantic Web Conference ISWC 2006, (November 2006), November 2006, Athens, GA, USA.
C. Ullrich, and A. Chen. Proceedings of the World Conference on Educational Multimedia, Hypermedia & Telecommunications (ED-MEDIA 2004), page 4059--4063. AACE, (2004)