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LDC-E1302 — Contrary unless-then under a norm head

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# LDC-E1302 — Contrary unless-then under a norm head

## What it means

A rule that concludes a duty or other position cannot carry a
contrary exception: `unless C then not P` would oppose a position that
has no complement. Only the bare proviso `unless C` is defined under a
norm head — it suspends the rule without deriving the opposite. The
contrary shape stops the compiler at the exception line.

The repair is to keep the bare proviso, or to move the contrary
conclusion into its own rule that derives the negated atom directly.
Opposition between positions travels through dedicated patterns, never
through an exception conclusion.

## Example

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

entity Tenant;
relation trig(t: Tenant) kind empirical;
relation done(t: Tenant) kind institutional;
relation harm(t: Tenant) kind institutional;
rule R defeasible {
    for t: Tenant;
    when trig(t);
    then duty D {
        bearer t;
        beneficiary t;
        achieve done(t) during [@2026-01-01, infinity);
    };
    unless harm(t) then not done(t);
}
```

## Fix

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

entity Tenant;
relation trig(t: Tenant) kind empirical;
relation done(t: Tenant) kind institutional;
relation harm(t: Tenant) kind institutional;
rule R defeasible {
    for t: Tenant;
    when trig(t);
    then duty D {
        bearer t;
        beneficiary t;
        achieve done(t) during [@2026-01-01, infinity);
    };
    unless harm(t);
}
rule Pay defeasible {
    for t: Tenant;
    when harm(t);
    then done(t);
}
```

## Engine message

The engine reports this in its own wording:

```text
example.law:17:5: error LDC-E1302: rule "R": the contrary `unless … then …` form under a norm head is undefined (§107.1): a position has no complement — position opposition goes via §105.1 patterns. Keep the bare `unless C` proviso or move the contrary conclusion to a separate rule
```

## Related

- [Defeasible rules and unless](/constructs/rule-defeasible-unless/) — provisos and their conclusions.
- [Duties](/constructs/duty/) — what a duty head must carry.