If someone buys a bundle with books A and B in it, they all future
updates to books A and B.

If a book C is added to the bundle, or if one of the books which was
in the bundle (say B) is removed from the bundle, this only affects
future purchasers of the bundle.  The bundle functions as a purchase
of each book in the bundle at the time.

If any reader has an issue with a purchase (of a bundle or anything else), we're happy to help.  They can email hello@leanpub.com

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