sclass OrderedMultiIterator<A extends Comparable<A>> extends ItIt<A> {

  srecord noeq Stream<A extends Comparable<A>>(Iterator<A> iterator, A element) is Comparable<Stream<A>> {

    public int compareTo(Stream<A> stream) {

      ret element.compareTo(stream.element);

    }

    bool advance() {

      if (!iterator.hasNext()) false;

      element = iterator.next();

      true;

    }

  }

  new ArrayList<Stream<A>> streams;

  bool sorted;

  void add(Iterable<A> it) {

    add(iteratorOrNull(it));

  }

  void add(Iterator<A> iterator) {

    if (iterator == null || !iterator.hasNext()) ret;

    A element = iterator.next();

    streams.add(new Stream<A>(iterator, element));

    sorted = false;

  }

  void addAll(Iterable<? extends Iterator<A>> iterators) {

    fOr (it : iterators) add(it);

  }

  public bool hasNext() {

    ret nempty(streams);

  }

  public A next() {

    if (!sorted) {

      sortInPlace(streams);

      set sorted;

    }

    var stream = first(streams);

    A element = stream.element;

    if (!stream.advance())

      streams.remove(0);

    else {

      // put stream in the right place

      int i = 0, n = streams.size()-1;

      Stream<A> stream2;

      while (i < n && stream.compareTo(stream2 = streams.get(i+1)) >= 0)

        streams.set(i++, stream2);

      streams.set(i, stream);

    }

    ret element;

  }

}