LDC-E1308 — Noncanonical defeater spelling

What it means

This warning family has two cases: a defeat clause with a strength other than defeater (the strength is corrected), and defeater with a then head (the head attacks the written polarity). The message distinguishes them. default is not a supported rule strength and is refused with E0201; it is not a third interpretation of this warning.

A defeater attacks an inference; it does not derive one. Its head names the attacked conclusion, and the canonical spelling is the defeat clause — a then part on a defeater reads “derive” while executing “block”, with polarity taken literally. The compiler keeps the node but warns at the strength, so the author can move the head into the canonical shape.

The repair is to spell the attack: replace the then part with defeat over the attacked atom. Note the polarity trap the message names — then not X attacks candidates for not X, not for X.

Example

Arxo Law

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

relation p(x: Text) kind institutional;
relation q(x: Text) kind institutional;
rule X1 defeater {
    for v0: Text;
    when p(v0);
    then q(v0);
}

Fix

Arxo Law

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

relation p(x: Text) kind institutional;
relation q(x: Text) kind institutional;
rule Base defeasible {
    for v0: Text;
    when p(v0);
    then q(v0);
}
rule X1 defeater {
    for v0: Text;
    when p(v0);
    defeat q(v0);
}

Engine message

The engine reports this in its own wording:

Output

example.law:7:9: warning LDC-E1308: rule "X1": defeater strength with a `then` part (§95.3); the defeater head names the ATTACKED inference, not the derived one — the canonical form is `defeat <atom>`. `then not X` attacks candidates for `not X`, not for `X`

Related

• Defeasible rules and unless — defeaters and the attacks they spell.
• Priority — ordering the inferences that survive attack.