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Anything, but the most useful are the specific ones: a chapter whose explanation did not land, an exercise whose review felt wrong, a diagram that broke on your phone. What you were doing and what happened instead is enough.

Disagreement is welcome. If an explanation here contradicts what you have seen in production, we would rather hear it than have it quietly noted and never said.

Signed in, we already have your address and can write back. Signed out, this is anonymous — which is fine, and sometimes the point. If you want an answer, the Contact page has an address that reaches a person directly.