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    Close to the start it says:

    White can open the game in any of twenty different ways, and black can respond with twenty of its own moves, creating 4,000 possible sequences after the first full turn.

    By my reckoning 20 times 20 is 400, not 4000.

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      Thank you! You are correct. I’m surprised I didn’t spot this. I checked Silver’s book and that section is reprinted there too.

      I’ve fixed this and the new version is being deployed.