 There are close parallels between changes in norms and changes in beliefs. In order to apply a norm system with conflicting norms to a particular situation, some of the norms may have to be ignored. The problem how to prioritize among conflicting norms is similar to the selection of sentences for removal in belief contraction (Hansson and Makinson 1997). The AGM model was in fact partly the outcome of attempts to formalize changes in norms rather than beliefs (Alchourrón and Makinson 1981). In spite of this, authors who apply the AGM model to norms have found it in need of rather extensive modifications to make it suitable for that purpose. Hence, in their model of changes in legislation, Governatori and Rotolo (2010) introduced an explicit representation of time in order to account for phenomena such as retroactivity.