static <A> ItIt<A> mapLike transitiveHullIteratorOfFunction(IF1<A, Cl<A>> f, Set<A> initial, O... _) {

  Set<A> seen = similarEmptySet(initial);

  // Make pool of iterators

  final new LinkedList<Iterator<A>> pool;

  pool.add(iterator(initial));

  ret iteratorFromFunction(new F0<A> {

    int n = 0;

    public A get() {

      while ping (!empty(pool)) {

        Iterator<A> it = first(pool);

        if (!it.hasNext()) continue with removeFirst(pool);

        // Get entry and check if seen already

        A entry = it.next();

        if (!seen.add(entry)) continue;

        // found new entry - return and schedule for further analysis

        Collection<A> newStuff = cast callF(f, entry);

        if (nempty(newStuff))

          pool.add(iterator(newStuff));

        ++n;

        ret entry;

      }

      null;

    }

  });

}