 DEL-style model-changing operators have been applied by a number of researchers to the study of preferences, preference change, and related notions. We refer the reader to  Appendix M  for further information, which mentions the work of van Benthem et al. (2009), van Benthem and Liu (2007), Liu (2008), Yamada (2007a,b, 2008), van Eijck (2008), van Eijck and Sietsma (2010), van Benthem, Girard, and Roy (2009c), and Liu (2011). 