http://doi.acm.org/10.1145/181550.181564 db/journals/sigmod/BunemanLSTW94.html Peter Buneman Leonid Libkin Dan Suciu Val Tannen Limsoon Wong 87-96 sigmodR/23-1/P087.pdf 1994 23 SIGMOD Record 1 db/journals/sigmod/sigmod23.html#BunemanLSTW94 http://www.acm.org/sigmod/record/issues/9403/Comprehension.ps