static <A> L<A> sortedByField(Collection<A> c, final S field) {
  L<A> l = new ArrayList(c);
  sort(l, new Comparator<A>() {
    public int compare(A a, A b) {
      ret cmp(getOpt(a, field), getOpt(b, field));
    }
  });
  ret l;
}

static <A> L<A> mapMethodLike sortedByField(S field, Collection<A> c) {
  ret sortedByField(c, field);
}