static <A, B> L<B> lookupAllOpt(Map<A, B> map, Collection<A> l) {
  new L<B> out;
  if (l != null) for (A a : l)
    addIfNotNull(out, map.get(a));
  ret out; 
}

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