Arc Skills / Requirements

Compare two versions of a requirements specification

Arc Skills compares two named specification revisions using stable IDs and controlled definitions. It returns a count-reconciled change register with candidate downstream impacts and unresolved identity matches.

Use this skill

Use Arc Skills to compare the two supplied specification revisions.

Inputs:
- Both complete scoped revisions with stable IDs, text, and locators
- Controlled glossary for each revision and any approved renumbering map
- Linked tests, allocations, interfaces, and supplier commitments for impact candidates

Return before/after excerpts, change classes, reconciled counts, impact candidates, and unresolved identity matches.
Match stable IDs first. Do not classify uncertain renumberings as confirmed additions or deletions

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What you provide and what you get

Inputs and outputs
What you have	How it is used	What you get
Two controlled revisions	Preserves exact text and source locators	Added, removed, changed, and unchanged register
Glossary and renumber map	Catches semantic change outside sentence text	Identity and meaning decisions
Downstream links	Locates likely review targets	Candidate follow-up list

Three records and a changed glossary

Illustrative engineering example.

Revision A has R-5, R-6, and R-7. Revision B has R-5, R-6, and R-70; R-70 may be a renumbering of R-7, but no approved map exists.

A R-5: Valve shall close within 5 s.  B R-5: Valve shall close within 2 s.
A/B R-6: The indicator shall show closed state.
A R-7: The log shall retain 30 entries.  B R-70: The log shall retain 30 entries.
Glossary: closed = limit-switch asserted (A); closed = seat contact confirmed (B).

Three records and a changed glossary — illustrative output
ID / identity	Class	Consequence and next check
R-5	Material: 5 s → 2 s	Review design margin and test acceptance for tighter bound.
R-6	Text unchanged, meaning material	Glossary changes observable closed state; inspect linked evidence.
R-7 ↔ R-70	Unresolved renumber candidate	Identical wording is not identity proof; request approved map.

Each revision contains three records. Two stable IDs match and both require impact review; one record in each revision remains unmatched pending the map. Reporting R-7 as deleted and R-70 as new would overstate the evidence.

The changed glossary makes R-6 materially different despite identical text. The register identifies affected tests as review candidates, not as automatically stale or formally changed.

Reconcile identity, then meaning

Record both source revisions and scoped record totals.
Match stable IDs, then present possible renumberings for human confirmation.
Compare actor, trigger, mode, comparator, limit, and controlled definitions.
Reconcile every input record to a class and list downstream review candidates.

Questions about this task

Can identical text prove a renumbering?

No. It is a candidate match; source lineage or an approved renumber map is needed.

Is a glossary-only edit editorial?

It can change the required observable or acceptance boundary. Inspect its use before classifying the requirement as unchanged.

Sources and further reading

NASA Systems Engineering Handbook: Lifecycle and requirements guidance; the example decisions are illustrative.