code: func (f *fragment) Checksum() []byte {
  	h := xxhash.New()
  	for _, block := range f.Blocks() {
  		_, _ = h.Write(block.Checksum)
  	}
  	return h.Sum(nil)
  }
docs: // Checksum returns a checksum for the entire fragment.
  // If two fragments have the same checksum then they have the same data.