code: private FastSet convert(IntSet c)
    {
      if (c instanceof FastSet) {
        return (FastSet) c;
      }
      if (c == null) {
        return new FastSet();
      }
  
      FastSet res = new FastSet();
      IntIterator itr = c.iterator();
      while (itr.hasNext()) {
        res.add(itr.next());
      }
      return res;
    }
docs: Convert a given collection to a {@link FastSet} instance