http://doi.acm.org/10.1145/140979.140996 Christian S. Jensen James Clifford Shashi K. Gadia Arie Segev Richard T. Snodgrass A Glossary of Temporal Database Concepts. 35-43 sigmodR/21-3/P035.pdf 1992 21 SIGMOD Record 3 db/journals/sigmod/sigmod21.html#JensenCGSS92