code: func (b *itemBundle) Digest(h crypto.Hash) (isolated.HexDigest, int64, error) {
  	a := h.New()
  	cw := &iotools.CountingWriter{Writer: a}
  	if err := b.writeTar(cw); err != nil {
  		return "", 0, err
  	}
  	return isolated.Sum(a), cw.Count, nil
  }
docs: // Digest returns the hash and total size of the tar constructed from the
  // bundle's items.