Yannis Tzitzikas Anastasia Analyti Nicolas Spyratos Compound Term Composition Algebra: The Semantics. 58-84 http://springerlink.metapress.com/openurl.asp?genre=article&issn=0302-9743&volume=3360&spage=58 2005 journals/jods/2005-2 J. Data Semantics 2 db/journals/jods/jods2.html#TzitzikasAS05