http://doi.acm.org/10.1145/141356.141362 db/journals/sigmod/EliassenK91.html Frank Eliassen Randi Karlsen Interoperability and Object Identity. 25-29 sigmodR/20-4/P025.pdf 1991 20 SIGMOD Record 4 db/journals/sigmod/sigmod20.html#EliassenK91