// 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 true {
    if (max != null && n >= max) ret seen;
    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 seen;
}