Michael Benedikt Guozhu Dong Leonid Libkin Limsoon Wong Relational Expressive Power of Constraint Query Languages. 1-34 1998 45 J. ACM 1 db/journals/jacm/jacm45.html#BenediktDLW98 http://doi.acm.org/10.1145/273865.273870 JACM/45/P0001.pdf