// trace_out collects what the function returns
// returns hashset including input
sclass TransitiveHull<A> implements Steppable {
  IF1<A, Iterable<A>> f;
  new Set<A> seen;
  new LinkedList<Iterator<A>> pool;
  Int max;
  
  *(IF1<A, Iterable<A>> *f, A starter) { add(starter); }
  
  void add(A a) {
    pool.add(singletonIterator(a));
  }
  
  public bool step() {
    if (max != null && l(seen) >= max) false;
    if (empty(pool)) false;
    Iterator<A> it = first(pool);
    if (!it.hasNext()) {
      removeFirst(pool);
      true;
    }
    process(it.next());
    true;
  }
  
  void process(A entry) {
    if (!seen.add(entry)) ret;

    // found new entry - return and schedule for further analysis
    Iterable<A> newStuff = f.get(entry);
    onGotCollection(entry, newStuff);
    if (nempty(newStuff))
      pool.add(iterator(newStuff));
  }
  
  swappable void onGotCollection(A entry, Iterable<A> newStuff) {
  }
}