static <A, B> Pair<L<B>, L<A>> lookupAllOpt2(Map<A, B> map, Collection<A> l) {
  new L<B> out;
  new L<A> notfound;
  if (l != null) for (A a : l) {
    B b = map.get(a);
    if (b != null) out.add(b); else notfound.add(a);
  }
  ret pair(out, notfound);
}

static <A, B> Pair<L<B>, L<A>> lookupAllOpt2(Collection<A> l, Map<A, B> map) {
  ret lookupAllOpt2(map, l);
}