A natural liberalization of Datalog is used in the Distributed Knowledge Authorization Language (DKAL). We show that the expressive power of this liberal Datalog is that of existential fixed-point logic. The exposition is self-contained.
@article{arxiv.0811.0977,
title = {Two Forms of One Useful Logic: Existential Fixed Point Logic and Liberal Datalog},
author = {Andreas Blass and Yuri Gurevich},
journal= {arXiv preprint arXiv:0811.0977},
year = {2008}
}