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;
    }
  });
}

Began life as a copy of #1023767