Ordinary algebraic equality is not a sound rewrite principle for measurement-bearing expressions. Reuse of the same observation matters, and division can make algebraically equal forms differ on where they are defined. We present a unified semantics that tracks both provenance and definedness. Token-sensitive enclosure semantics yields judgments for one-way rewriting and interchangeability. An admissible-domain refinement yields a domain-safe rewrite judgment, and support-relative variants connect local and global admissibility. Reduction theorems recover the enclosure-based theory on universally admissible supports. Recovery theorems internalize cancellation, background subtraction, and positive-interval self-division. Strictness theorems show that reachable singularities make simplification one-way and make common-domain equality too weak for licensed replacement. An insufficiency theorem shows that erasing token identity collapses distinctions that definedness alone cannot recover. All definitions and theorems are formalized in sorry-free Lean 4.

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