IEnumerable <Tuple <int, string> > Tuples(IndexPQBase <string> pq)
 {
     foreach (int i in pq)
     {
         yield return(new Tuple <int, string>(i, pq.KeyOf(i)));
     }
 }
        IEnumerable <string> Strings(IndexPQBase <string> pq)
        {
            foreach (int i in pq)
            {
                yield return(i.ToString());

                yield return(pq.KeyOf(i));
            }
        }