http://www.loc.gov/mods/v3 http://www.loc.gov/standards/mods/v3/mods-3-2.xsd
Nested relations and complex objects in databases
S. Abiteboul, P.C. Fischer, H.-J. Schek, (eds.).
"Selected papers from the workshop Theory and applications of nested relations and complex objects held in Darmstadt (West Germany) on April 6-8, 1987"--P. [iii].