CanUndo
fCouldUndo = editor.CanUndo();
if (fCouldUndo != fEditor.CanUndo())
update.AddBool("can_undo", fEditor.CanUndo());
if (!CanUndo())
bool CanUndo() const;
fUndoMenuItem->SetEnabled(fEditor.CanUndo());
bool CanUndo() { return !fUndos.IsEmpty(); }
fUndoItem->SetEnabled(fSudokuView->CanUndo());