core/docs/decisions/ADR-0138-comparative-reference-layer.md

ADR-0138 — Comparative-Reference Layer

Status: Draft (design-only) Date: 2026-05-23 Parent: ADR-0136 Informed by: ADR-0136.S3-post-rescan, ADR-0136.S2-post-rescan (ADR-0137 deferral note) Supersedes (in scope): ADR-0137 — closed without merging; reopened here in a lighter, shape-driven form

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Context

The S.x corridor is bounded. Each phase ships one closed regex shape and yields 0–1 admissions + ~1 barrier shift. After S.0/S.1/S.2/S.3 the GSM8K probe sits at 3/50 admitted, wrong=0, with the next-largest blockers fragmented across heterogeneous shapes:

compound_statement      5  (heterogeneous — see ADR-0136.S.3 deferred list)
novel_initial_form      5  (S.4 candidate)
fraction_operand        4  ← target of this ADR
novel_initial_verb      4
compound_comparative    3  ← target of this ADR
conditional_question    3
context_filler          3  (correctly refused)

The seven cases under fraction_operand + compound_comparative are not seven separate regex shapes. They share a common deep structure: a value-bearing phrase whose value is relative to either a numeric anchor in the same sentence or a referenced quantity from a previous sentence.

Per-case decomposition:

Case	Shape	Anchor source
0005	decrease to 3/4 of its temperature + current temperature is 84	sentence-internal numeric
0029	keyboard cost three times greater than the cost of the mouse + mouse cost $16	sentence-internal numeric
0041	75% of the 2nd pan (after 2 pans…16 pieces per pan)	sentence-internal referent
0043	father gave twice as much as her mother (after mother gave $4)	prior-sentence quantity
0010	Marion has 1/4 more than what Yun currently has, plus 7	prior-sentence derived quantity
0004	Half of the kids… + 1/4 of the kids going to soccer camp… + 750 kids…in the afternoon	inverse: known sub-quantity, unknown total
0036	as much again as Wednesday, Thursday and Friday combined	multi-term aggregate reference

0005 and 0041 admit sentence-internal grounding. 0029 is the forcing case for cross-sentence binding — its comparative ("three times greater than the cost of the mouse") in one sentence resolves against a literal anchor ("the mouse cost $16") in a later sentence, with no ambiguity. 0043 requires a similar chain (twice as much as her mother after mother gave $4). 0010 is genuinely ambiguous English — its expected dataset answer requires a non-standard parse of "1/4 more than X"; this ADR refuses on it (see §Scope). 0004 and 0036 require additional machinery (inverse arithmetic, multi-term aggregation) and are out of scope.

Why this ADR, not S.x. S.x ships one regex shape per phase. The comparative-reference cluster is one deep structure with multiple surface shapes. Treating each surface as a separate S.x phase wastes effort and produces a forest of overlapping regexes. A small typed layer that represents the deep structure once and consumes multiple surfaces is the right unit of work.

Why not ADR-0137 as drafted. The closed-and-deferred ADR-0137 proposed a generic DeferredCandidate / BindingProof apparatus for any kind with open slots. Its deferral reason stands: subsuming the S.1/S.2 short-circuits (which are joins of grounded candidates, not deferred bindings) wouldn't have used the machinery. Now that gsm8k-0010 has surfaced a real deferred-binding case, the right response is not to revive the generic apparatus. It is to ship a typed reference layer with one shape — QuantityReference — that specifically handles cross-sentence quantity lookup, and to keep the machinery as light as the actual problem.

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Decision

1. Three typed phrase structures, no generic deferred-candidate apparatus

Introduce three frozen, slotted dataclasses representing the deep structure of comparative/fractional phrases:

@dataclass(frozen=True, slots=True)
class FractionOperand:
    """A fraction-shaped operand: 1/4, half, 75%, 3/4, etc."""
    numerator: int                  # e.g. 1
    denominator: int                # e.g. 4
    of_target: ReferenceTarget | None  # e.g. "the 2nd pan", "the kids"
    source_span: str

@dataclass(frozen=True, slots=True)
class ComparativeOperand:
    """A comparison: 'three times greater than X', '1/4 more than X',
    'twice as much as X', 'half as long as X'."""
    direction: Literal["more", "less", "times_greater", "times_as_much",
                       "fraction_more", "fraction_less"]
    modifier: FractionOperand | int | float  # the multiplier or addend
    anchor: ReferenceTarget                  # what's being compared to
    source_span: str

@dataclass(frozen=True, slots=True)
class ReferenceTarget:
    """An anchor: either a literal numeric value visible in the same
    sentence, or a typed reference to a quantity introduced earlier."""
    kind: Literal["literal", "quantity_reference"]
    literal_value: int | float | None          # set when kind == 'literal'
    quantity_reference: QuantityReference | None  # set when kind == 'quantity_reference'

@dataclass(frozen=True, slots=True)
class QuantityReference:
    """A typed cross-sentence reference: 'what Yun currently has',
    'the cost of the mouse', 'the 2nd pan', 'her mother [gave]'."""
    entity: str | None             # named entity (Yun), or None for "the X" descriptors
    descriptor: str | None         # 'the 2nd pan', 'the cost of the mouse'
    qualifier: str | None          # 'currently', 'initially'
    source_span: str

    @property
    def needs_binding(self) -> bool:
        return True

These types are descriptive only at the first pass. They carry no admissibility check on their own; they are slots that the binding pass (§3) closes.

2. Extractor responsibilities (first pass)

Three new extractor functions in generate/math_candidate_parser.py, each producing typed phrases rather than full candidates:

• extract_fraction_operands(sentence) → list[FractionOperand]
• extract_comparative_operands(sentence) → list[ComparativeOperand]
• (QuantityReference is constructed inside the above two when their anchor is non-literal.)

These extractors emit phrases. They do not produce CandidateInitial or CandidateOperation directly. Promotion happens in the binding pass.

3. The binding pass (lighter than ADR-0137's draft)

A single new control-flow stage between first-pass extraction and the existing Cartesian product. Its only job: resolve QuantityReference slots against prior-sentence grounded candidates, then promote fully-grounded ComparativeOperand / FractionOperand phrases into CandidateInitial (when they describe an entity's quantity) or slot-fillers for question candidates (when they describe a value asked about).

The binding pass:

1. Iterates over ComparativeOperand + FractionOperand phrases in sentence order.
2. For each phrase with a non-literal anchor, looks up the QuantityReference against the cumulative grounded-candidate set from earlier sentences.
3. Resolves only when the reference is unique — single matching (entity, descriptor) → single quantity. Ambiguous references refuse the resolution (drop, not best-effort).
4. Computes the resolved value via the comparative/fractional rule:
   • times_greater(N, anchor): anchor * (N + 1) (interpretation pinned by ADR-0138; N times greater is ambiguous in English and we pick one closed reading — see §Non-negotiables).
   • times_as_much(N, anchor): anchor * N
   • fraction_more(F, anchor): anchor * (1 + F)
   • fraction_less(F, anchor): anchor * (1 - F)
   • more(N, anchor): anchor + N
   • less(N, anchor): anchor - N (refuses on negative result)
5. Promotes the resolved value into the existing CandidateInitial path via the same round-trip filter.

No DeferredCandidate type. No BindingProof type. The pass is a deterministic lookup-and-promote operation. Replay determinism comes from the same JSON-sort-keys serialization the cognition eval already uses.

4. Sentence-internal grounding (no binding pass needed)

When ComparativeOperand.anchor.kind == "literal", the value is extracted in the first pass without involving the binding pass. This handles the 0005 / 0029 cases (sentence-internal numeric anchor) and 0041 (sentence-internal referent + same-sentence quantity).

5. ADR-0137 supersession

The closed ADR-0137 is superseded in scope by this ADR. The deferral note in ADR-0136.S2-post-rescan.md stands; the binding machinery this ADR proposes is the answer to its "reopen criterion." This ADR is lighter because it commits to one typed reference shape (QuantityReference) rather than a generic DeferredCandidate apparatus.

The S.1/S.2 short-circuits in parse_and_solve stay as joins of grounded candidates. They are not retrospective binding and this ADR does not subsume them. (The canonical-runner staleness issue is fixed separately by teaching the runner to score short-circuit admissions.)

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Scope

In scope (this ADR + its implementation phases)

• FractionOperand, ComparativeOperand, ReferenceTarget, QuantityReference type machinery
• First-pass extractors for the closed surface set:
  • <N> times greater than <anchor>
  • <N> times as much/many/long as <anchor>
  • <fraction> more than <anchor> (where <fraction> ∈ {1/2, 1/4, 3/4, half, twice} or <N>%)
  • <fraction> less than <anchor>
  • <fraction> of <descriptor>
  • <N>% of <descriptor>
• Binding pass: sentence-order lookup of QuantityReference against prior grounded candidates with unique-match gating
• Probe lane RC1_comparative_reference with the three cleanly in-scope cases (0029, 0041, 0010) plus negative probes (0005 — see below — 0004, 0036, 0043)
• ADR-supersession note added to ADR-0136.S2-post-rescan.md acknowledging that 0137's reopen criterion is met by 0138

Out of scope (deferred to follow-up ADRs)

• Inverse arithmetic (case 0004): "750 kids are afternoon, 1/4 are morning → total = 1000" requires solving for the unknown total from a known sub-quantity. Different machinery (inverse fraction inference).
• Multi-term aggregate references (case 0036): "as much again as Wednesday, Thursday and Friday combined" requires summing multiple prior quantities through a single reference. Probably ADR-0139.
• Self-reference (case 0005): "decrease to 3/4 of its temperature" — note this case is in the in-scope set as a negative probe: the binding pass must refuse on self-references (its resolves to the same entity whose value is being computed, yielding a circular binding). Confirm in the lane that 0005 still refuses.

Wait — re-examining 0005: "Addison mountain's temperature will decrease to 3/4 of its temperature. If the current temperature is 84 degrees, what will the temperature decrease by?" The anchor is sentence-internal — "the current temperature is 84." The "its" refers to "Addison mountain's temperature" which is given as 84. So 0005 is actually in scope under sentence-internal grounding once the question-form is parsed. Reclassified to in-scope. Negative probes remain 0004 and 0036.

Cases this ADR is expected to admit

Case	Expected outcome	Mechanism
0005	admit (answer=21)	sentence-internal fraction-of-anchor + question-resolution rule
0029	admit (answer=64)	cross-sentence QuantityReference resolution: comparative in sentence 2 ("three times greater than the cost of the mouse") resolved against sentence 3's literal ("the mouse cost $16")
0041	partial — likely still refuses	needs sentence-internal referent resolution; depends on whether _INITIAL_HAS_RE or its sibling grounds "2 pans" with a per-pan quantity
0043	partial — needs chain (mother + father + total + purchase)	the comparative twice as much as her mother resolves cleanly; total-aggregation across the rest is multi-term, out of scope

Honest expected admission delta: +1 to +2 (0005, 0029 at high confidence; 0041 and 0043 partial credit only if the linguistic structure composes with existing extractors).

On gsm8k-0010 (the case that surfaced the forcing function). The forcing case for cross-sentence binding turned out to be 0029, not 0010. 0010's sentence 2 ("Marion has 1/4 more than what Yun currently has, plus 7") has a genuinely ambiguous English reading. The dataset's expected answer is 9, which requires parsing the phrase as (1/4) * Yun + 7 = (1/4)*8 + 7 = 9 — treating "1/4 more" as "1/4 [of what Yun has]" rather than "1/4 more than [Yun's quantity]." The natural English reading (Yun * (1 + 1/4) + 7 = 17) disagrees with the dataset.

Per non-negotiable #2 of this ADR, 0010 must refuse. Multiple plausible parses of the same surface yield different numeric values; the binding pass refuses rather than picks one. 0010 is reclassified from "expected admission" to "negative probe" — it stays refused, and the reason (ambiguity) becomes a regression assertion in the lane.

This is the right outcome under refusal-first discipline, and it proves that the cross-sentence binding case must be justified by an unambiguous example. 0029 is that example.

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Non-negotiables

1. wrong == 0 remains a hard CI gate. Across the binding eval lane and the GSM8K probe.
2. Ambiguous reference → refuse the binding. If QuantityReference has multiple matching prior candidates with different values, the resolution refuses; the problem refuses; no best-effort.
3. Pinned English-reading convention for "N times greater." English usage is ambiguous (some readers: anchor + anchor * N; others: anchor * N). This ADR pins the additive reading (anchor * (N + 1)) and documents the choice in the ADR + the lane test. If a downstream case appears that requires the other reading, refuses-then-revisit; do not silently pick the other.
4. No new short-circuit path in parse_and_solve. Promoted candidates flow through the existing graph machinery.
5. No solver/graph/verifier changes. Linguistic layer ends at candidate emission.
6. Determinism. Same input → byte-equal BindingPassResults, byte-equal promoted-candidate set, byte-equal report.json.
7. Closed surface vocabulary. All comparative + fractional surface markers are enumerated. No wildcards. New markers require extending the closed set (in the same PR as the test case that forces them).

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Probe set

Required admissions

Case	Source	Why this exercises the layer
0005	GSM8K	sentence-internal fraction-of-anchor + literal-value path
0029	GSM8K	cross-sentence QuantityReference resolution; the forcing case for the binding pass

Required refusals (negative probes)

• gsm8k-0010 — ambiguous English (1/4 more than X parses as either X * (1 + 1/4) or (1/4) * X); the binding pass must refuse per non-negotiable #2
• gsm8k-0004 — inverse arithmetic (must continue to refuse pending ADR-0139)
• gsm8k-0036 — multi-term aggregate reference (must continue to refuse pending ADR-0139)
• A synthetic ambiguous-reference case where two prior sentences introduce the same (entity, unit) with different values — must refuse (uniqueness gate)

Axis cases (synthetic, ≥15)

Closed combinations of the surface vocabulary × anchor kinds:

• <frac> of <literal> ≥3
• <frac> of <reference> ≥3
• <N> times as much as <literal> ≥3
• <N> times greater than <reference> ≥3
• <frac> more than <reference> ≥3 (covers 0010's shape)

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Phased rollout

One branch per phase. The 2026-05-23 worktree-race lesson stands.

Phase	Deliverable	Branch
0138.D	This ADR	docs/adr-0138-comparative-reference-layer (this PR)
0138.A	generate/comparative/{types,extractors}.py + tests for the type machinery and the closed surface vocabulary (no binding pass yet)	feat/adr-0138-types-and-extractors
0138.B	Binding pass + QuantityReference resolution + 0010 admission + 0005/0029 admissions	feat/adr-0138-binding-pass
0138.C	Eval lane RC1_comparative_reference + probe set	feat/adr-0138-rc1-lane

Each phase is its own PR. Each must hold wrong == 0, preserve all existing axis lanes (S.1, S.2, S.3, S.4 if landed), and not regress the rescan-v3 invariants.

After 0138.B lands, run a rescan v4 to record barrier shifts and update the post-rescan ADR.

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Open questions (deferred to implementation ADRs, not this one)

• Promotion of fractional-of-set phrases. "Half of the kids" with no prior quantification of "the kids" produces a refusal in 0138.B (no resolvable reference). Case 0004's full solution requires inverse arithmetic on this same shape. Future ADR-0139 (inverse arithmetic) will compose with this layer.
• Pronoun resolution depth. "Marion has 1/4 more than what Yun currently has" names Yun explicitly. Cases like "he has 1/4 more than what she has" need pronoun resolution against the prior-actor set. Out of scope; refuse on pronoun-anchored references until a future ADR.
• Telemetry. Whether QuantityReference resolution events emit into the existing turn-event telemetry sink. Probably no for now; cognition runtime is not consuming math problems on the hot path.

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Consequences

• The forcing function for retrospective binding (denied at #204 closure, surfaced at #208 rescan v3) is met by a typed reference layer rather than a generic deferred-candidate apparatus. Smaller, more specific, easier to audit.
• The S.x corridor continues for shape-specific extensions (novel_initial_form, novel_initial_verb, the remaining compound_statement shapes). 0138 is parallel to S.x, not its replacement.
• The corpus of comparative + fraction-of-quantity problems is large in GSM8K's full set (not just the 50-case train sample). This layer's value compounds as the probe widens beyond the train sample.
• The pinned "N times greater = N+1 multiplier" reading is a CORE convention. If we later encounter a case that requires the other reading, the lane test will flag it and the closed-set decision becomes revisitable rather than buried.

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What this ADR is not

• Not a code change. No generate/comparative/ module exists at merge time.
• Not a generic deferred-candidate framework. It commits to one typed reference shape (QuantityReference) and one binding rule (unique-match cross-sentence lookup).
• Not a replacement for the S.x corridor. The two are parallel.
• Not a license to add semantic guessing. Every comparative, fractional, and reference shape is a closed-set surface pattern with a deterministic resolution rule.
• Not in conflict with CLAUDE.md. The binding pass is exactly the shape of "comprehend → recall → think" — hold a typed reference, bind it deterministically when the prior quantity is grounded, refuse when ambiguous.