UndoesPrevious
if (edit->UndoesPrevious(top.Get())) {
virtual bool UndoesPrevious(const UndoableEdit* previous);
virtual bool UndoesPrevious(const Command* previous);
if (command->UndoesPrevious(top)) {
virtual bool UndoesPrevious(const Command* previous);
if (command->UndoesPrevious(top)) {