S. Heymans, D. Nieuwenborgh, and D. Vermeir. Transactions on Computational Logic, 9 (4):
1-53(August 2008)ACM, 2008. This is the author's version of the work. It is posted here by permission of ACM for your personal use. Not for redistribution. The definitive version was published in Transactions on Computational Logic, VOL 9, ISS 4, (August 2008) http://doi.acm.org/10.1145/1380572.1380575.