MathProblemGraph.__post_init__ now raises MathGraphError when two
InitialPossession entries share the same (entity, unit) key but
declare different quantity values.
Pre-fix behavior surfaced by 2026-05-28 ADR-0174 Phase 3 post-merge
diagnostic: math_solver.solve() line 207 used last-write-wins dict
assignment when consolidating initial state. Two contradictory
inputs would silently overwrite without trace:
'Sam has 5 marbles. Sam has 3 marbles. How many marbles does Sam have?'
→ returned 3.0 (wrong=0 violation: definite answer from
contradictory input)
Post-fix: same input refuses with 'no branch produced a solvable
graph' — refusal-preferring discipline as wrong=0 doctrine requires.
Identical duplicates (same value) are admitted as redundant (no
contradiction). Different units for same actor admitted. Different
actors for same unit admitted. Single-value cases (the dominant
real-world pattern) unchanged.
This is an extraction-layer hazard discovered while investigating
Phase 3b scope: Phase 3b compound-clause held hypotheses would
emit multiple CandidateInitial entries per sentence, exercising
exactly this consolidation path. Fixing the silent overwrite NOW
ensures Phase 3b admission doesn't silently produce wrong answers.
Acceptance:
- 4 new tests in TestContradictoryInitialPossessionsRefuse
- 165/165 test_math_problem_graph tests pass (was 161/161)
- Smoke 67/67, packs 141/141 unchanged
- train_sample 3/47/0 unchanged (no real case exercised the
overwrite — but the hazard was latent)
References: CLAUDE.md §Lookback Review Discipline (the doctrine
that surfaced this), CLAUDE.md §Non-Negotiable Field Invariant
(make illegal states difficult to represent).
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