code: func (it *ReaderIteratorImpl) Next() (v int64, err error) {
  	nextValues, err := it.nextN(1)
  	if err != nil {
  		return 0, err
  	}
  	assert(len(nextValues) > 0)
  	return nextValues[0], nil
  }
docs: /*
  Lucene(Java) manipulates underlying LongsRef to advance the pointer, which
  can not be implemented using Go's slice. Here I have to assume nextN() method
  would automatically increment the pointer without next().
  */