A conditional strategy prescribes actions according to specified observations, states, or histories. The conditioning information must be available to the player when the action is chosen; the rule does not require conscious calculation.
In an iterated prisoner's dilemma, Tit for tat conditions on the opponent's previous action, whereas a grim trigger strategy remembers whether any defection has occurred. Both can choose the same action in the first round and then respond differently to the same most recent move because their retained histories differ. A finite-state strategy records the relevant past information in an automaton state. In an extensive-form game, the prescription must be constant across an information set, since its nodes cannot be distinguished when choosing an action.
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