Compositions accept implicitSection on a sections model

implicitSection exists on the sections model (packages/types/src/content-model.ts): when no section heading is found, all children become one section instead of preamble. The composition parser does not accept the key, so itinerary, whose conditional content model only exists to add it, cannot be expressed. Accepting it removes that rune's need for a conditional model (SPEC-169 D2, D3).

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Criteria completion

Tracking started Oct 24 — check back for trends.

Relationships

ID:SPEC-169Status:accepted

What the composition parser must accept, and what belongs elsewhere

ID:WORK-648Status:done

each iterates the items of a list field

Priority:mediumComplexity:simpleMilestone:v0.42.0

ID:SPEC-169Status:accepted

What the composition parser must accept, and what belongs elsewhere

ID:v0.42.0Status:complete

v0.42.0 — What a composition can express

main current done

changeset-release/main done claude/v0-42-0-planning-t31f5e done main done claude/v0-42-0-planning-ata7nd ready

Acceptance Criteria

A composition accepts implicitSection on a sections model, and its attribute keys must equal emitAttributes' keys, with an error naming the mismatch otherwise
With no section heading, all content becomes one item of the sections slot, and $each is bound the same way as for a real section, asserted in a test
The error for a conditional content model names SPEC-169's alternatives: implicit sections, list each and templates
The composed-runes guide documents implicitSection

Approach

In a composition there is no emitTag, so the implicit section is an ordinary item of the sections slot and its attributes supply the $each values (D3).
The error text for a conditional model is shared with WORK-648's and D5's alternatives. D5 (templates covering several values) waits on SPEC-168's templates; name it in the message only once it exists, or phrase the message so it does not promise it.

References

SPEC-169: D2, D3.

Resolution

Completed: 2026-10-25

Branch: claude/v0-42-0-planning-t31f5e

What was done

packages/runes/src/lib/resolver.ts: one implicitSection() path. Without emitTag, it now returns a section entry (body resolved by sectionModel, declared attributes under IMPLICIT_SECTION_KEY). The emitTag path is unchanged.
composition.ts: parses implicitSection, and checkImplicitSection requires its keys to equal emitAttributes' (implicit-section-mismatch). itemsOf binds $each from the implicit attributes. The conditional-model error names the alternatives without promising D5's templates.
Guide section on implicitSection; tests in composition-models.test.ts.