 holds at a frame \(F\) if and only if the core is empty, and a hybrid logic extension can tell us whether a specific outcome belongs to the core. If \(F\) is based on a truly playable effectivity function we already have a normal form game version of the core: an outcome such that no coalition is together able and willing to deviate from, not taking into account what the others do. However Nash Equilibrium fixes a profile of strategies, such that no player is able and willing to deviate from there. In other words it requires the notion of best response for a player with respect to a given profile.