J. Hidders, and J. Sroka. On the Move to Meaningful Internet Systems: OTM 2008, volume 5331 of Lecture Notes in Computer Science, Springer Berlin / Heidelberg, (2008)
J. Paredaens, and D. Suciu. Proc. of the Thirteenth ACM SIGACT-SIGMOD-SIGART Symposium on Principles of Database Systems (SIGMOD 1994), page 201--209. (1994)