= Imperfect information
In an <extensive-form game>, a player has imperfect information when some decision <information set> contains more than one node. The player cannot distinguish those histories at the time of acting. This does not itself imply failure of <perfect recall>: a player can remember all their own earlier information and actions while never having observed an opponent's hidden choice. Conversely, merging histories in which the player's own earlier actions differ violates <perfect recall>.
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