[Rook]

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The SAGE System is based on what mathematicians call an "equilibrium strategy," which is a strategy that cannot be "beaten" in the following sense: If either player deviates from the equilibrium, his expectation will go down (and thus, in a two-player game, his opponent's expectation must go up).

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Direct from the Cardplayer article on the SAGE system. If you read it carefully, it makes the assumption that both players are using the SAGE (or equilibrium strategy). So if one person deviates, that person's expectation decreases.

However, the scenario you put forth is that SB is already deviating from it. His expectation is, of course, way down if he's pushing that tight. If you already know he's deviating, and you know in what way.. then it would be foolhardy to not adjust accordingly. Again, that doesn't make SAGE exploitable, or wrong, it just means it won't always give you the most $EV move.