code: func (iter *ReferenceSliceIter) Next() (*plumbing.Reference, error) {
  	if iter.pos >= len(iter.series) {
  		return nil, io.EOF
  	}
  
  	obj := iter.series[iter.pos]
  	iter.pos++
  	return obj, nil
  }
docs: // Next returns the next reference from the iterator. If the iterator has
  // reached the end it will return io.EOF as an error.