This material has been published in the Journal of Computer and System Sciences, Vol. 60, No. 3, 2000, pp. 660-716, the only definitive repository of the content that has been certified and accepted after peer review. Copyright and all rights therein are retained by Academic Press. This material may not be copied or reposted without explicit permission. Copyright © 2000 by Academic Press. This paper is available at the IDEAL Online Library.