structure ('Undo') _before_ it is changed in some edit operation.
Obviously, the stored ranged should be as small as possible. However, it
there is a lower limit: The StableDocIterator pointing stored in the undo
-struct must be valid after the changes, too, as it will used as a pointer
+class must be valid after the changes, too, as it will used as a pointer
where to insert the stored bits when performining undo.
*/
-struct Undo {
+class Undo {
+public:
/// This is used to combine consecutive undo recordings of the same kind.
enum undo_kind {
/**