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LDC-E1342 — Priority role the definition never exports

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# LDC-E1342 — Priority role the definition never exports

## What it means

A priority may order generated rules through the roles their template
exports: `Instance.role` names the rule the definition assigned to that
role. A role the definition never exported resolves to no rule, and an
edge into the void would read as law as written — so the compiler
stops at the side instead.

The repair is to export the role from the definition, or to point the
priority at a role that is exported. Every side then lands on a rule
that exists.

## Example

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

entity Person;
entity Choice;
entity SpecialChoice : Choice;
entity Other;
const Selected: SpecialChoice = entity_ref("urn:choice:selected");
const Wrong: Other = entity_ref("urn:choice:wrong");
relation eligible(p: Person, choice: Choice) kind empirical;
relation applicable(p: Person, choice: Choice) kind institutional;
expansion fixed {
    params { subject: binder; choice: value; ground: relation(subject, choice); result: relation(subject, choice); }
    exports { outcome = self/result; }
    emit rule self/result defeasible {
        for subject;
        when ground(subject, choice);
        then result(subject, choice);
        scope from self; effective from self; labels from self; source from self;
    }
}
expand fixed Result {
    label en official "Fixed option";
    bind subject = person: Person;
    choice = Selected;
    ground = eligible;
    result = applicable;
    effective [@2026-01-01, @2027-01-01);
}
priority P1 {
    prefer Result.outcome over Result.ghost;
    reason explicit_exception;
}
```

## Fix

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

entity Person;
entity Choice;
entity SpecialChoice : Choice;
entity Other;
const Selected: SpecialChoice = entity_ref("urn:choice:selected");
const Wrong: Other = entity_ref("urn:choice:wrong");
relation eligible(p: Person, choice: Choice) kind empirical;
relation applicable(p: Person, choice: Choice) kind institutional;
expansion fixed {
    params { subject: binder; choice: value; ground: relation(subject, choice); result: relation(subject, choice); }
    emit rule self/result defeasible {
        for subject;
        when ground(subject, choice);
        then result(subject, choice);
        scope from self; effective from self; labels from self; source from self;
    }
}
expand fixed Result {
    label en official "Fixed option";
    bind subject = person: Person;
    choice = Selected;
    ground = eligible;
    result = applicable;
    effective [@2026-01-01, @2027-01-01);
}
```

## Engine message

The engine reports this in its own wording:

```text
example.law:32:32: error LDC-E1342: priority P1: role Result.ghost is not exported by the definition (§279.1.6)
```

## Related

- [Priority](/constructs/priority/) — edges and the rules they may order.
- [LDC-E1336](/diagnostics/LDC-E1336/) — an instance must resolve to exactly one definition.