Georg Gottlob Christoph Koch Monadic Datalog and the Expressive Power of Languages for Web Information Extraction. 17-28 2002 conf/pods/2002 PODS http://doi.acm.org/10.1145/543613.543617 http://www.acm.org/sigs/sigmod/pods/proc02/papers/017-GottlobKoch.pdf db/conf/pods/pods02.html#GottlobK02