We present a modal logic for reasoning about what groups of agents can bring bout by collective action. Given a set of states, we introduce game frames which associate with every state a strategic game among the agents. Game frames are essentially extensive games of perfect information with simultaneous actions, where every action profile is associated with a new state, the outcome of the game. A coalition of players is effective for a set of sates X in a game if the coalition can guarantee the outcome of the game to lie in X.
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