// f : A -> Collection<A>

// trace_out collects what the function returns

static <A> Set<A> mapLike transitiveHullOfFunction_multipleInputs(O f, Cl<A> inputs, O... _) {

  final HashSet<A> seen = asHashSet(inputs);

  final Int max = cast optPar max(_);

  Map trace_out = cast optPar trace_out(_);

  // Make pool of iterators

  final new LinkedList<Iterator<A>> pool;

  fOr (A input : inputs)

    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;

}