static <A> Iterator<A> chainIterators(Collection<Iterator<A>> l) {
  final L<Iterator<A>> _l = new LinkedList(l);
  ret iteratorFromFunction(func -> A {
    while (nempty(_l)) {
      if (first(_l).hasNext())
        ret first(_l).next();
      _l.remove(0);
    }
    null;
  });
}