Priority

In one sentence: a priority declaration states which of two opposite defeasible inferences wins when both apply. The author uses it when a general and a special norm collide on one case: without a declared edge, two incomparable candidates are both preserved (Contradiction), and no “obvious speciality” decides anything on its own.

The language specification treats priority as an explicit order between opposite defeasible inferences: when both apply, the higher one wins, and without a declared edge two incomparable candidates are both preserved. The exception forms that remove support without an edge live on the defeasible-rules page.

OverviewBoundariesCorpus formsPitfalls

1. When to use and when not to

Instead	Selection rule
priority vs unless in a rule	A local exception inside one article (“except in cases…”) is an unless clause: it expands into a defeater or a contrary rule and needs no separate declaration. A collision of two standalone norms from different articles is priority with a reason. Criterion: the exception knows only its own rule — unless; the norm knows its competitor by name — priority
priority vs a strict negative head	A prohibition without exceptions beats any permission on its own: strict opposite support defeats a defeasible candidate without an edge. priority is needed only between two defeasible candidates (or a rule over a defeater). Priority over a strict rule is a rejection LDC-E4105
reason lex_specialis vs reason lex_posterior	A special norm beats a general one, a later one beats an earlier one — but neither the number of conditions nor the date of adoption decides anything on its own: the ground is written as an explicit reason. Date and act level are facts for a priority policy with when, not automaticity
An explicit edge vs silent Contradiction	Incomparable candidates are both preserved: Contradiction is a substantive answer — “conflict unresolved” — not an error. An edge is declared when the law truly prefers one side; keeping Contradiction is lawful when the collision is genuine

2. Minimal example

Package research.priority.special_over_general: a general norm admits a registered vendor to trading; a special one refuses on licence revocation. Without the edge both inferences are incomparable; the edge grounded in lex_specialis makes the special norm the winner:

Arxo Law

rule GeneralTrade defeasible {
    for v: Vendor;
    when registered_trader(v);
    then may_trade_on_square(v);
}
rule RevocationRefusal defeasible {
    for v: Vendor;
    when licence_revoked(v);
    then not may_trade_on_square(v);
}
priority RevocationOverGeneral {
    prefer RevocationRefusal over GeneralTrade;
    reason lex_specialis;
}

Case facts: registered_trader + licence_revoked (both with origin case_input). Query: evaluate truth(may_trade_on_square(...)).

Actual engine answer:

Output

law test research.priority.special_over_general: мир research.priority.special_over_general
  ok   [research.priority.special_over_general#authored] tests/01-special-wins.lawtest / urn:query:research-priority-01
  ok   [research.priority.special_over_general#authored] tests/02-general-alone.lawtest / urn:query:research-priority-02
итого: 2 проверено, 2 прошли, 0 не прошли, 0 не исполнены; код 0

law engine check — check OK, no warnings. The first test answers Refuted (RevocationRefusal ≻ GeneralTrade); the second, with the single fact registered_trader, — Established.

Sensitivity is proven by a pair of packages: research.priority.no_priority_both holds the same two rules without a priority declaration, and the same two-fact case answers Contradiction (law test 2/2). The single difference between the packages is the edge: it alone changes Contradiction to Refuted. Removing the licence_revoked fact in either package gives Established — the examples are not vacuous.

Nearest wrong outcome: expecting “speciality is visible by number of conditions” so no edge is needed. The core does not count speciality: without the edge the answer is Contradiction, and that is correct behaviour, not an engine defect.

3. Example by domain

• Law: package kz.corpus.administrative_code (Administrative Offences Code of Kazakhstan), emergency vehicles and signals — a prefer … over … edge with reason lex_specialis: the railway-crossing offence (art. 607 part 1) displaces the general prohibitory-signal offence (art. 599 part 1); the in-file comment directly records “without the edge the defeater would have given Contradiction” (see the corpus forms).
• Law: package kz.corpus.administrative_code (Administrative Offences Code of Kazakhstan), sanctions — a series of edges of the percentage fine over the part 2 art. 44 limits: one special norm beats four general limits.
• International law: package intl.labour.c138 (ILO Convention No. 138, minimum age), final provisions — a contrary unless … then not … proviso: the refusal form of an exception without a separate priority declaration.
• Teaching case: package research.priority.no_priority_both — the same conflict without the edge: an executable demonstration that silence gives Contradiction, not victory to the “obviously special” norm.

4. How the engine answers

Table — actual runs of this section’s teaching packages:

Facts	Question	Answer	Why
registered_trader, licence_revoked, edge present	may_trade_on_square	Refuted	RevocationRefusal ≻ GeneralTrade
same facts, no edge	may_trade_on_square	Contradiction	candidates incomparable
only registered_trader	may_trade_on_square	Established	second rule not applicable

Additionally, checked behaviours of the earlier edition (no dedicated tests in this section’s packages; not fixed by a run here):

Facts	Question	Answer	Why
registered_trader, sells_prohibited_goods	may_trade_on_square	Refuted	strict support beats a defeasible one without priority
registered_trader, sanitary_violation	may_trade_on_square	Not established, not refuted	the defeater removed support; the complement is not derived
same + sells_essential_goods	may_trade_on_square	Not established, not refuted	the priority condition not established — the defeater beats both candidates
same + state_of_emergency	may_trade_on_square	Established	EssentialGoodsTrade ≻ SanitaryBlock; a weak candidate does not hinder the strong one
same, priority cut	may_trade_on_square	Not established, not refuted	counterfactual: the priority over the defeater alone decides the outcome

The unless proviso scenarios (bare and contrary forms) live on the defeasible-rules page: a proviso’s expansion belongs to the rule, not to priority.

• Priority is a strict partial order: the transitive closure counts; absence of a path is incomparability, not “first/last wins”.
• A preserved conflict is part of the answer: both sides’ candidates, priority edges, the reason for unresolvability.
• In why_not, a defeated candidate names the winner and the edge; on Contradiction — records the absence of a path between candidates.
• A conditional priority applies to a pair of applications only when its when is established; the corpus does not use it.
• The short edge form — priority Name: A > B reason ground;: the same declaration in one line. The choice between block and one-line form is a matter of readability: with a label and comments the block is clearer, otherwise the line is shorter and inferior in nothing.
• Priority of a rule over a defeater is the only lawful case where the sides’ heads match in polarity: the general rule survives under the defeater, and a weak candidate of the same side does not hinder the strong one. The defeated GeneralTrade application does not spoil the surviving EssentialGoodsTrade inference — defeat is pinpoint, not contagious.
• A ring closed at runtime from conditional edges gives the conflict group PRIORITY_CONFLICT: both sides preserved, no tie-break.

5. Common mistakes

1. “The special norm wins anyway” — without the edge, Contradiction (pitfalls, item 1).
2. Expected Refuted, got Not established, not refuted: a bare defeater/unless removes support without asserting the opposite (pitfalls, item 2).
3. unless on a strict rule — LDC-E4110 (pitfalls, item 3).
4. Priority over a strict rule — LDC-E4105 (pitfalls, item 4).
5. Two prefer clauses in one block or a block without prefer — LDC-E0201; a side naming no rule — LDC-E4108/LDC-E1318 (pitfalls, item 5).
6. A priority ring — LDC-E4104 already at check (pitfalls, item 6).
7. An edge between same-polarity heads — warning LDC-E2142: such a pair is not opposite, and the edge expresses nothing (pitfalls, item 7).

6. References

• Neighbour pages: defeasible rules (unless provisos, defeater), strict rules, negation.
• The defeater-vs-priority tutorial lives in the tutorials catalogue.
• The pitfalls page lists the diagnostics (LDC-E4110, LDC-E4105, LDC-E0201, LDC-E4108/LDC-E1318, LDC-E4104, LDC-E2142) with wrong forms and fixes.

7. Grammar excerpts

Show syntax reference

Conflict clauses inside a rule (the defeat_clause and unless_clause productions; the full rule_decl grammar lives on the strict-rules page; the unless expansion lives on the defeasible-rules page):

Grammar

defeat_clause           = "defeat", proposition, ";" ;
unless_clause           = "unless", [ identifier, "when" ], formula, [ "then", proposition ], ";" ;

A priority declaration (the priority_decl, priority_item productions; exactly one edge per declaration; a priority policy is the priority_policy_decl production):

Grammar

priority_decl           = "priority", identifier,
                          ( "{", { priority_item }, "}"
                          | ":", reference, ">", reference,
                            "reason", qualified_name, ";" ) ;
priority_item           = "prefer", reference, "over", reference, ";"
                        | "when", meta_formula, ";"
                        | "reason", qualified_name, ";"
                        | source_anchor_item
                        | label_item
                        | metadata_item ;