LDC-E2142 — A priority edge that settles nothing

What it means

A priority between two rules settles a genuine conflict: the two rules must conclude opposite things about the same predicate, so that preferring one over the other changes the outcome. When both rules conclude the same thing — literals of one predicate with the same polarity — there is nothing to settle, and the edge can never change anything under any arguments. The compiler keeps the package green but warns at the priority.

This is a warning, not a refusal: the package still checks. Give the two rules complement heads so the edge has a conflict to settle, or drop the edge if no conflict was meant.

Example

Arxo Law

language "law.core" version "0.2";
package demo.diagnostics version "0.1.0";
namespace "urn:law:demo:diagnostics";

entity Person;
relation member(p: Person) kind empirical;
relation rate(p: Person, r: Integer) kind institutional;
rule A defeasible { for p: Person; when member(p); then rate(p, 15); }
rule B defeasible { for p: Person; when member(p); then rate(p, 20); }
priority P: A > B reason lex_specialis;

Fix

Arxo Law

language "law.core" version "0.2";
package demo.diagnostics version "0.1.0";
namespace "urn:law:demo:diagnostics";

entity Person;
relation member(p: Person) kind empirical;
relation rate(p: Person, r: Integer) kind institutional;
rule A defeasible { for p: Person; when member(p); then rate(p, 15); }
rule B defeasible { for p: Person; when member(p); then not rate(p, 20); }
priority P: A > B reason lex_specialis;

Engine message

The engine reports this in its own wording:

Output

example.law:10:10: warning LDC-E2142: priority P is powerless: rule heads `A` and `B` are literals of one predicate `rate` of one polarity, while §105 makes applications opposite only under complement heads; §107 gives such a pair no victory under any arguments. The working form is a guard in the general rule body together with an explicit defeater (§107 item 3 keeps the «правило над дефитером» edge alive) or a §165 decision table

Related

• Priority — how conflicts between rules are settled.
• Defeasible rules — how a rule yields to an exception.