static <A> Comparator<A> combineComparators(final Comparator<A>... l) {
  ret new Comparator<A> {
    public int compare(A a, A b) {
      for (Comparator<A> c : l) {
        int i = c.compare(a, b);
        if (i != 0) ret i;
      }
      ret 0;
    }
  };
}