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 TreeSet<Stream<A>> streams;
  
  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));
  }
  
  void addAll(Iterable<? extends Iterator<A>> iterators) {
    fOr (it : iterators) add(it);
  }
  
  public bool hasNext() {
    ret nempty(streams);
  }
  
  public A next() {
    var stream = first(streams);
    A element = stream.element;
    
    streams.remove(stream);
    if (stream.advance())
      streams.add(stream);
      
    ret element;
  }
}

Began life as a copy of #1034675