code: func (s *DiffIndexScanner) prepareScan() bool {
  	s.next, s.err = nil, nil
  	if !s.from.Scan() {
  		s.err = s.from.Err()
  		return false
  	}
  
  	return true
  }
docs: // prepareScan clears out the results from the last Scan() loop, and advances
  // the internal scanner to fetch a new line of Text().