Function verify_change 

pub fn verify_change(
    doc: &mut AutoCommit,
    hash: ChangeHash,
    registry: &ActionRegistry,
) -> Result<Verified, ActionError>

Replay the action the change hash records on the document it was authored against, and check that it produces exactly that change: same operations, same actor, time and message, so the same hash. This is what a device that has the app does with every change it receives, beside the relay’s grant check: an author granted an action who wrote something else inside its storage units is caught here.