A.21:4.8 - Crossing and semantic-Bridge boundary

For a structural crossing, receive the exact changed-binding and crossing facts from E.18. Add a crossing check only when its criterion applies. SquareLaw is required only when the E.18 crossing rule for that case requires it.

A structural crossing does not imply an F.9 semantic Bridge. Add an F.9 Bridge, bounded-use claim, reliance, optional Bridge Card, or optional CL only when the separate semantic-correspondence relation and downstream use obtain. A gate whose decision does not rely on semantic correspondence carries none of this apparatus. Do not encode absent Bridge material as mandatory fields with none values.