Function offer_back 

pub(crate) fn offer_back(
    qt: &CxxQtThread<Sioul>,
    shared: &Arc<Shared>,
    line: String,
    back: impl Fn(&CxxQtThread<Sioul>, &Arc<Shared>) -> String + Send + Sync + 'static,
)

A change made already (a drag): “Undo” stays offered for a moment, as for mail, and back puts back what was there.