static <A, B> L<Pair<A, B>> pairsSortedByAComparator(Collection<Pair<A, B>> l, final Comparator<A> c) {
  ret sortedByComparator(l, new Comparator<Pair<A, B>>() {
    public int compare(Pair<A, B> a, Pair<A, B> b) {
      ret c.compare(a.a, b.a);
    }
  });
}