Logic-Lens — Semantic Diff

When to Use

Use this skill when you need compare two code versions for semantic equivalence via semi-formal tracing of both versions side-by-side. Trigger when the user shares a refactor, rewrite, migration, or A/B implementation and wants to confirm behavior is unchanged — "did I break anything", "is this equivalent", "are...

Setup

Use lazy loading per ../_shared/common.md §13:

1. Read ../_shared/common.md only for language, Iron Law, Verdict header, scope routing, Remedy discipline, config fields, and loading budget.
2. Read only the relevant step in logic-diff-guide.md as you reach it.
3. Load ../_shared/logic-risks.md, ../_shared/semiformal-guide.md, ../_shared/semiformal-checklist.md, and ../_shared/report-template.md on demand when the current step needs them.

Process

Step 0. Language + scope routing. Detect language per common.md §1. Confirm two versions are provided. If only one version, switch to logic-review.

Step 1. Identify the shared specification (guide Step 1) — what inputs should both versions handle; what outputs/side effects are expected. If the user states the refactor intentionally changed behavior in a specific area (e.g., "I changed the error path to raise instead of returning None"), record that as a declared spec change and treat divergences within that area as expected. Flag only divergences outside the declared change as findings.