// a task to finish, then execute another one which waits
            // for the initial task...
            // In order to work around that problem, we override the
            // invokeAll method, so that whenever the method is called,
            // the pool core size will be incremented before submitting
            // all the tasks, then the thread will block waiting for all

    private final int mask;

    SubSet(ImmutableMap<E, Integer> inputSet, int mask) {
      this.inputSet = inputSet;
      this.mask = mask;
    }

    @Override
    public Iterator<E> iterator() {
      return new UnmodifiableIterator<E>() {
        final ImmutableList<E> elements = inputSet.keySet().asList();
        int remainingSetBits = mask;

        @Override