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I don't have strong opinions about FlagErrorWrap also being available. Would FlagErrorf("%w", err) be a functional equivalent to the wrap function?

No, because fmt.Errorf("%w", err) is not the identity function. (I had had the same question.)

I was initially inclined not to add the wrap function, but concluded that it is consistent with the abstraction. We already expose the concept of "the underlying error" through Unwrap; FlagErrorf("%w", err) doesn't let you control it.