Function undo_takes_back 

pub fn undo_takes_back(
    kind: ActionKind,
    change: ChangeKind,
    irreversible: bool,
) -> bool

Whether Undo takes back a value with these facts: it changes the document alone (ActionKind::Undoable), as a person’s step (ChangeKind::is_step), and loses nothing Undo cannot bring back (Action::irreversible). The one rule every host asks, whatever the value’s type.