static <A> L<A> concatLists(Iterable<A>... lists) {
  new L<A> l;
  if (lists != null) for (Iterable<A> list : lists)
    addAll(l, list);
  return l;
}

static <A> L<A> concatLists(Collection<? extends Iterable<A>> lists) {
  new L<A> l;
  if (lists != null) for (Iterable<A> list : lists)
    addAll(l, list);
  return l;
}