 Our presentation of the DEL approach to belief change will follow Baltag and Smets (2008b), so all theorems and definitions in the remainder of Section 4 are due to them unless otherwise noted. Their work is closely linked with the work of van Benthem (2007), Board (2004), Grove (1988), and others. For an alternative approach based on Propositional Dynamic Logic, we refer the reader to van Eijck and Wang (2008). 