One of the merits of the notion of sequential equilibrium is the emphasis on out-of- equilibrium beliefs-that is, on beliefs (about past and future play)at information sets that should not be reached if given equilibrium is played. The key insight of extensive-form analysis is that out-of-equilibrium beliefs deter. mine equilibrium behavior. For instance, consider the simple two-stage entry deter- rence game in which potential entrant decides whether to enter a market or stay out, and the incumbent decides whether to fight or acquiesce after the entrant's move
