Logic Diff

---
name: logic-diff
description: "Compare two code versions for semantic equivalence via semi-formal tracing of both versions side-by-side."
risk: safe
source_type: community
date_added: 2026-07-01
license: MIT
---

# 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.

**Step 2. Build independent premises for each version** (guide Step 2) — apply the Premises Construction Checklist to Version A and Version B separately.

**Step 3. Trace both versions for the common case** (guide Step 3) — parallel trace, same input, note the first divergence if any.

**Step 4. Trace boundary cases** (guide Step 4) — empty/null/zero, max/min, error inputs, first/last of collections. Start with at most three highest-risk boundary scenarios unless the user asks for exhaustive equivalence or the shared specification requires more.

**Step 5. Identify and classify semantic divergences** (guide Step 5) — each divergence is a finding with Premises → Trace → Divergence → Trigger → Remedy and an L-code.

**Step 6. Equivalence verdict** (guide Step 6) — one of: `✅ Semantically Equivalent`, `⚠️ Conditionally Equivalent` (state the condition precisely), `❌ Semantically Divergent`.

**Step 7. Output** (guide Step 7) — Report Template with the Verdict header per `common.md` §5; localize headers if the user wrote in Chinese. **Format is mandatory even for trivially short snippets: every divergence finding MUST use the five labeled fields (Premises / Trace / Divergence / Trigger / Remedy); never substitute with a plain paragraph or table.**

**Mode line in report:** `Semantic Diff` (Chinese: `语义对比`).

## Limitations

- Use this skill only when the task clearly matches its upstream source and local project context.
- Verify commands, generated code, dependencies, credentials, and external service behavior before applying changes.
- Do not treat examples as a substitute for environment-specific tests, security review, or user approval for destructive or costly actions.