 Contraction is the fundamental form of belief change studied by AGM. There is no mystery in how to define expansion, and revision is usually defined derivatively via the  Levi identity (1977): \(\bB*p = (\bB \div \neg p) + p.\) Setting the mold for future work (Gärdenfors 1988, Gärdenfors and Rott 1995), Alchourrón, Gärdenfors and Makinson (1985) proceed axiomatically: they postulate several principles that every rational contraction operation must satisfy. Fundamental to AGM theory are several representation theorems showing that certain intuitive constructions give rise to contraction operations satisfying the basic postulates and conversely, that every operation satisfying the basic postulates can be seen as the outcome of such a construction. See the entry on  belief revision,  Huber (2013a), or Lin (2019) for an excellent introduction to these results. 