sS addToLocalMechList_uniq(S listName, S text) {
  LS existing = tlft(localMechList_opt_raw_fresh(listName));
  LS newLines = listMinusSet(lines(text), existing);
  if (empty(newLines)) ret "No change";
  appendToLocalMechList(listName, lines(newLines));
  ret n2(l(newLines), "line") + " added";
}