 Action model (Baltag, Moss, and Solecki 1998, 1999; see also Baltag and Moss 2004). Other names in the literature: “event model” or “update model”. Given a set of formulas \(\Lang\) and a finite nonempty set \(\sA\) of agents, an action model is a structure   \[ A=(E,R,\pre) \]   consisting of