function getChangesAdded(backend1, backend2) {
  return backendState(backend2).getChangesAdded(backendState(backend1))
}
Returns all changes that are present in `backend2` but not in `backend1`.
Intended for use in situations where the two backends are for different actors.
To get the changes added between an older and a newer document state of the same
actor, use `getChanges()` instead. `getChangesAdded()` throws an exception if
one of the backend states is frozen (i.e. if it is not the latest state of that
backend instance; this distinction matters when the backend is mutable).