static Triple<L<S>> attractToTwoLists1(L<S> things, L<S> l1, L<S> l2) {
  Set<S> set1 = asCISet(l1), set2 = asCISet(l2);
  new L<S> a; new L<S> b; new L<S> undecided;
  for (S s : unnull(things)) {
    L<S> pot = doubleBoolSwitch(
      contains(set1, s), a,
      contains(set2, s), b,
      undecided);
    pot.add(s);
  }
  ret triple(undecided, a, b);
}