// f : A -> Collection<A>

// trace_out collects what the function returns

// returns hashset including input

static <A> Set<A> mapLike transitiveHullOfFunction(O f, A input, O... _) {

  final Set<A> seen = optPar(_, seen := (Set) lithashset(input));

  final Int max = cast optPar max(_);

  Map trace_out = cast optPar trace_out(_);

  // 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);

    mapPut(trace_out, entry, newStuff);

    if (nempty(newStuff))

      pool.add(iterator(newStuff));

    ++n;

  }

  ret seen;

}

static <A> Set<A> transitiveHullOfFunction(IF1<A, Cl<A>> f, A input, O... _) {

  ret transitiveHullOfFunction((O) f, input, _);

}