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");

}

S undoActions(S s) {

  if "undo" ret singleUndo();

  if "anything to undo"

    ret empty(undoList) ? "Nothing to undo" : n2(undoList, "undoable action");

  if "clear undos" ret "OK" with clearUndos();

  null;

}

// API

UndoableWithAnswer lastUndo() {

  ret dm_q(() -> last(undoList));

}