LDC-E1303 — Defeater without head

For LLMs5 sections

What it means

A defeater with neither a defeat clause nor a then part attacks nothing and derives nothing, so the compiler skips the node and warns. In practice the parser reports the same headless rule first as LDC-E0201, at the same position, so this warning is never the first diagnostic on a plain check: the error is listed before the warning.

The repair is to give the defeater its attack: add a defeat clause over the attacked atom, alongside a defeasible rule that produces a candidate for the attack to meet.

Example

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 R defeater {
    for v0: Text;
    when p(v0);
}

Fix

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 R defeater {
    for v0: Text;
    when p(v0);
    defeat q(v0);
}

Engine message

The engine reports this in its own wording:

example.law:7:6: warning LDC-E1303: defeater "R": without a `defeat` clause and a then-part; node is skipped

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