Defeasible rules: boundaries

What the construct does NOT do in 0.2: limits, closed lists, neighbouring constructs, and the selection rule.

OverviewBoundariesCorpus formsPitfalls

Does not do

• Does not infer the complement. A defeater and a bare unless remove support; after defeat the answer is Not established, not refuted, not Refuted. Negation must be asserted in the contrary form or by a not-head rule.
• Does not defeat the strict. Nobody defeats strict support: neither defeater (LDC-E4112) nor priority (LDC-E4105). Two strict rules with opposite heads give a preserved Contradiction.
• Does not count speciality or seniority. Number of conditions and date of adoption decide nothing: the ground is given by an explicit relation with a reason. Neither the “latest” nor the “first” rule wins on its own.
• Does not see the power effect. The effect of a power is invisible to a same-round defeater: an unless condition reading an effect-reachable predicate silently does not fire. Write the defeater on the raw case fact.
• Does not split the contrary proviso on a norm head. unless C then not duty … is undefined and rejected at lowering: a position has no complement. A bare proviso on a norm head is lawful and gives a defeated position.

Reserved and closed

• Priority is a strict partial order: irreflexive, transitive, acyclic. A static cycle is a rejection LDC-E4104, including prefer A over A; a ring of conditional priorities surfaces at runtime as PRIORITY_CONFLICT with answer Contradiction.
• priority is exactly one prefer A over B edge per declaration; without prefer or on repeat — LDC-E0201. A side naming no rule — LDC-E4108/LDC-E1318: such priority never applies.
• defeat — one per defeater block (LDC-E1329 on repeat); defeat not p(x) is rejected by the parser (LDC-E0201).
• Repeated scope/effective/defeat in one block — LDC-E1329.

Neighbours and selection rule

• unless vs priority: a proviso removes support inside the norm; priority resolves a collision of two opposite inferences. See the selection table on the defeasible-rules page.
• unless vs a separate defeater: same article and same variables — a clause; another act, reuse, own priority — a separate rule.
• Bare vs contrary unless: Not established, not refuted silence vs established Refuted refusal.
• strict vs defeasible: no exceptions now or ever vs at least one conceivable one.