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LDC-E1205 — Counterfactual with the wrong shape

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# LDC-E1205 — Counterfactual with the wrong shape

## What it means

A counterfactual block is a fixed-shape what-if: exactly one target
being rewritten, exactly one cost function, and at least one mutable
reference the analysis may vary. The same strictness covers the sibling
name checks — an interpretation may only cite readings and names that
exist, and a context profile may only extend a profile that is
declared. A block that breaks the shape, or a reference that points at
nothing declared, stops the compiler at the offending line.

The repair is to restore the shape: one target, one cost, at least one
mutable entry, and references that name things the package declares.

## Example

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

entity Offence;
const Theft: Offence = Offence { };
relation listed(o: Offence) kind institutional;
function EditCost(cost: Decimal) -> Decimal = cost;
counterfactual IfX {
    target listed(Theft);
    target listed(Theft);
    mutable listed;
    cost EditCost;
}
```

## Fix

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

entity Offence;
const Theft: Offence = Offence { };
relation listed(o: Offence) kind institutional;
function EditCost(cost: Decimal) -> Decimal = cost;
counterfactual IfX {
    target listed(Theft);
    mutable listed;
    cost EditCost;
}
```

## Engine message

The engine reports this in its own wording:

```text
example.law:9:16: error LDC-E1205: counterfactual "IfX": exactly one `target` is required (§186)
```

## Related

- [Interpretations](/constructs/interpretation/) — readings and the names they may cite.
- [LDC-E1201](/diagnostics/LDC-E1201/) — the companion check for doubly declared names.