static <A> L<A> sortedByFields(Collection<A> c, final S... fields) {
  L<A> l = new ArrayList(c);
  sort(l, new Comparator<A>() {
    public int compare(A a, A b) {
      for (S field : fields) {
        int x = cmp(getOpt(a, field), getOpt(b, field));
        if (x != 0) ret x;
      }
      ret 0;
    }
  });
  ret l;
}