static <A, B> L<A> keysSortedByValues(final Map<A, B> map) {
  L<A> l = new ArrayList(map.keySet());
  sort(l, new Comparator<A>() {
    public int compare(A a, A b) {
      ret cmp(map.get(a), map.get(b));
    }
  });
  ret l;
}