static Collection<S> nlLogic_certainOperatorArgs(L<Exp> l, S... operators) {
  HashSet<S> set = lithashset(operators);
  new LinkedHashSet<S> out;
  for (Exp e : unnull(l)) {
    if (e cast Func)
      if (set.contains(e.name))
        out.add(nlLogic_text(e.arg));
  }
  ret out;
}