Willem Conradie Valentin Goranko Dimiter Vakarelov Elementary Canonical Formulae: A Survey on Syntactic, Algorithmic, and Model?theoretic Aspects. 17-51 2004 conf/aiml/2004 Advances in Modal Logic http://www.aiml.net/volumes/volume5/Goranko.ps db/conf/aiml/aiml2004.html#ConradieGV04