I'm still not sure how easy it will be to remove one type from a union. That would be a gamechanger in error-handling ergonomics.
There's a reason for it, which is that set union and intersection (types are just sets of properties) don't have unique inverses. (A∪B)∪B = A∪B. What would the inverse of ∪B be? One could argue for A or A∪B, but only A∪B is ⊇ A ∀ A, B; hence, the only general solution.
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Yes... but (unfortunately) I don't think that evidence is trivial in general. Err1 and Err2 may be large unions, and the evidence you construct at such time as the types are concrete isn't the same evidence you need in an abstract context.
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