Defeasible rules: 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. unless on a strict rule; defeater against a strict head

unless in rule … strict — rejection LDC-E4110 (exception nodes are not generated); a defeater whose target is produced only by strict rules or by nobody — rejection LDC-E4112. A strict rule has no exceptions by definition. Detection: law engine check fails. Fix: make the rule defeasible or move the applicability condition into the strict body.

1a. Repeated defeat in one block

Two defeat clauses in one defeater are a rejection LDC-E1329: one defeat target per block, like scope and effective. Detection: law engine check names the repeated clause.

2. The proviso does not bind all head variables

unless stall_reserved(s) on head may_use_stall(v, s) — rejection LDC-E4101 on the name <Rule>/unless/N: the body of the generated node is the condition C itself, the general rule’s when is not carried into it, and range restriction applies to the node separately. Detection: law engine check names the generated name. Fix: repeat the binding conjunct as the first premise of the clause (unless stall_requested(v, s) and stall_reserved(s);).

3. Expected Refuted, got Not established, not refuted

A bare unless and a separate defeater remove support without asserting the opposite. Refusal is written in the contrary form unless C then not A or as a rule with a not A head and priority. The difference is measured on the ILO No. 138 package (intl.labour.c138): the bare variant would have given “undetermined” where the law says “no”.

4. A defeater cancelled a valued literal and carried away another’s inference

defeat srednedneva(w, s, <computed-value>) on matching values removes EVERY candidate of the named literal, including other inferences: the result is empty, check and the evaluation document are clean, not a single issue anywhere. Detection: only by reading the answer — warning LDC-E2144 sounds when there are two or more defeasible producers of the predicate. Fix: cancel a valueless applicability relation, not a literal carrying a number.

5. Priority between same-polarity heads

Two inferences of one relation are not opposite under any arguments — the edge expresses nothing: warning LDC-E2142. The corpus still holds 46 such dead edges in 13 packages. Exception: an edge between links of an unless chain — it is kept alive by the closure, and LDC-E2142 no longer sounds on it.

6. Answer Contradiction although “it is clear which norm is special”

The core does not count speciality by number of conditions or date: declare priority with a reason. Priority over a strict rule is impossible here (LDC-E4105): source rank is defeasible + priority … reason lex_superior or priority policy, not strict.

7. Traps for AI agents

• A then { a; b; } block is accepted by the grammar but not lowered by the compiler: warning LDC-E1302, zero rules in the compiled output. Write separate then clauses — each becomes a separate head rule.
• Rule strength is explicit: use defeasible for a rule that may be defeated, strict otherwise.
• A head variable bound only by comparison or negation — LDC-E4101: add a positive literal that binds it.
• A keyword name (event, case, calendar, action) as a binder — LDC-E0203; quoted identifiers removed.
• Silent outcome: Not established, not refuted on an unfired rule is not refusal; Contradiction on two incomparable candidates is a preserved conflict, part of the answer.