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new L<UndoableWithAnswer> undoList;

void addUndo(UndoableWithAnswer undo) {
  if (undo == null) ret;
  undoList.add(undo);
  change();
  print("Undo added: " + undo);
}
void clearUndos q {
  if (empty(undoList)) ret;
  undoList.clear();
  change;
  print("Undo list cleared");
}

S singleUndo() {
  UndoableWithAnswer undo = popLast(undoList);
  if (undo == null) ret "Nothing to undo";
  change();
  ret or2(undo.undo(), "Last action undone");
}

// API

UndoableWithAnswer lastUndo() {
  ret dm_q(() -> last(undoList));
}