// f : A -> Collection<A>
static <A> Set<A> mapLike transitiveHullOfFunction(O f, A input, O... _) {
  final HashSet<A> seen = lithashset(input);
  final Int max = cast optPar max(_);
  
  // Make pool of iterators
  final new LinkedList<Iterator<A>> pool;
  pool.add(iterator((Iterable<A>) callF(f, input)));
  int n = 0;
    
  while ping (!empty(pool) && (max == null || n < max)) {
    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 seen;
}