Peter Buneman Wenfei Fan Jérôme Siméon Scott Weinstein Constraints for Semi-structured Data and XML. 47-45 2001 30 SIGMOD Record 1 http://doi.acm.org/10.1145/373626.373697 http://www.acm.org/sigmod/record/issues/0103/Leonid-DP.pdf db/journals/sigmod/sigmod30.html#BunemanFSW01