static <A, B> Comparator<Pair<A, L<B>>> lengthOfPairBListComparator() {
  ret new Comparator<Pair<A, L<B>>>() {
    public int compare(Pair<A, L<B>> a, Pair<A, L<B>> b) {
      ret cmp(l(a.b), l(b.b));
    }
  };
}