Lex specialis: the specific rule defeats the general

Area: Defeasible reasoning Teaches: resolving a same-strength norm conflict with #[defeats] — lex specialis, where a specific rule beats the general one it names, per-tuple, by an explicit checked edge rather than a pair of magic priority integers. Prerequisites: strict vs default vs defeater. Run: ox build examples/legal_priority_v0 && ox run-scenario examples/legal_priority_v0

Two norms conclude about the same head at the same strength: imported goods owe import duty (general); medical imports are exempt (specific). Without a superiority mechanism this conflict is unresolvable — both fire on a medical import and you get a contradiction. Lex specialis is the legal rule that the specific norm wins; Argon expresses it as one rule defeating another’s labeled clause.

What to read in statute.ar

The general norm is an overridable, labeled default. #[label(general)] is the handle a more specific rule will name:

#[default]
#[label(general)]
pub derive must_pay_duty(g) :- Imported(g);

The specific norm names the general clause and defeats it. The #[defeats] target is qualified by the label — must_pay_duty.general(g) — and resolves per-tuple against the bound g. It defeats the general duty clause for exactly the goods exempt derives, and no others:

#[defeats(must_pay_duty.general(g))]
pub derive exempt(g) :- MedicalGood(g);

This is the whole lex-specialis idea in two lines: the more specific rule (medical imports) defeats the more general one (imported goods), tuple by tuple. A non-medical import is untouched by the defeat and keeps owing duty.

Running it

The scenario declares four goods:

steel    imported=true   medical=false  → owes duty   (general default, unattacked)
insulin  imported=true   medical=true   → exempt      (specific defeats general)
gauze    imported=true   medical=true   → exempt      (specific defeats general)
widget   imported=false  medical=false  → no clause fires

so dutiable (the warranted must_pay_duty) returns 1 row — steel — while exemptions returns 2 rows, insulin and gauze. The two medical imports are dutiable under the general norm and exempt under the specific one; lex specialis subtracts them from the duty extent, leaving the single non-medical import.

This example is compiled and run in CI; the post-defeat must_pay_duty and exempt extents are pinned by a corpus test (oxc-runtime/tests/examples_corpus.rs), so the lex-specialis resolution can’t drift.