Paolo Atzeni Luca Cabibbo Giansalvatore Mecca Letizia Tanca The LOGIDATA+ Language and Semantics. 30-41 1993 books/sp/Atzeni93 LOGIDATA+: Deductive Databases with Complex Objects db/journals/lncs/lncs701.html#AtzeniCMT93 http://dx.doi.org/10.1007/BFb0021888