code: func (bg *Writer) Next() (int, error) {
  	if bg.closed {
  		return 0, ErrClosed
  	}
  	if err := bg.Error(); err != nil {
  		return 0, err
  	}
  
  	return bg.active.next, nil
  }
docs: // Next returns the index of the start of the next write within the
  // decompressed data block.