Presumptions and fictions: pitfalls

Updated 3 October 2026: concept, except_when, claim, definition … sufficient, and default strength have been removed, together with their special deprecation diagnostic. References below to the former behavior and corpus sources are historical. New source uses relation, unless, duty, a strict rule, and defeasible.

Wrong forms, silent outcomes, LDC-E diagnostics, typical mistakes of formalizers and AI agents, and how to detect them.

OverviewBoundariesCorpus formsPitfalls

1. The exception does not bind its variables

The range-restriction requirement applies to the generated rule wholly and separately: every variable of its head must be bound by a positive established-conjunct of the exception condition itself; bound only in the general rule’s when is not bound. The diagnostic is LDC-E4101 on the generated node’s name (<presumption>/R2, …). The practical fix is repeating the binding conjunct as the clause’s first premise: logically an identity (when the general rule fires the premise is already true), while the variable moves into bound inside the node with its own body.

2. A bare unless does not give Refuted

A bare clause generates a defeater: the application is beaten, there are no inferences, the answer is Not established, not refuted. Proven by the section’s run: case 02 of the innocence teaching package answers Not established, not refuted, not Refuted. Whoever expected established absence of innocence writes unless … then not innocent(p) (contrary form: a defeasible rule with direct priority). The mistake is silent: law test is green, the meaning is wrong.

3. unless on a strict rule is a static error

The author either makes the rule defeasible or moves the exact applicability condition into the strict body so no defeat is needed. For a genuinely strict contrary inference — a separate explicit strict rule; the linter demands a rationale. The AI agent appending unless to a strict rule “for completeness” gets a rejection, not a warning.

4. A fiction with an empirical head

A fiction infers an institutional fact; the compiler warns if the head relation has empirical kind. A fact about the world (“delivered”) is a rule; a legal equation (“counts as received”) is a fiction. A mixed-up kind is a silent level confusion: the inference looks like a world fact but proves as an institution.

5. Foreign clauses in a presumption body — a loud rejection

scope, effective, source anchors, and expansion metadata have no expansion in presumption semantics: in 0.1 the implementation must reject them statically (LDC-E0201), not lower the presumption without them. Same for fiction, classification, and definition: source anchors and metadata in their bodies are LDC-E0201. The rejection is loud by design: a presumption applied wider than written is worse than a rejection.

6. Rules and exception chains

Write exceptions with the unless clause; the order of multiple clauses defines the priority chain (§150). Choose a defeasible rule when its conclusion may be defeated.

7. A chain without an edge gives Contradiction

Two contrary clauses with one conclusion (unless C1 then not A; unless C2 then not A) share an identical head literal — that pair is not opposite to each other, and defeat gives it no victory. The chain edge carries order to the transitive closure, not to itself: without it, a case where only the later link fires answers Contradiction instead of Refuted. Detection: run a case with one late link; Contradiction where Refuted was expected is a lost edge.