code: func readChunk(file *os.File, offset int64, buf []byte) (err error) {
  	n, err := file.ReadAt(buf, offset)
  	if err != nil {
  		return
  	}
  	if n != ChunkSize {
  		return fmt.Errorf("Invalid read %v", n)
  	}
  	return
  }
docs: // Read a chunk of a file at `offset` so as to fill `buf`.