Turn informal requirements into quantified membership rules, construct languages, and prove the limits of a precisely stated expression grammar.
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Does a separate witness for each set give one witness shared by every set?
Membership specifications depend on the order and scope of their quantifiers.
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No. Two disjoint nonempty sets each have a member but have no common member. State which choices a witness may depend on.
An expression-limit theorem must cover every permitted syntax constructor.
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Prove the property for each base expression, then prove every constructor preserves it assuming the property for each arbitrary child. Checking finitely many expressions cannot replace these cases.