code: func AddTrytes(bndl Bundle, fragments []Trytes, offset int) Bundle {
  	for i := range bndl {
  		if i >= offset && i < offset+len(fragments) {
  			bndl[i].SignatureMessageFragment = Pad(fragments[i-offset], 27*81)
  		}
  	}
  	return bndl
  }
docs: // AddTrytes adds the given fragments to the txs in the bundle starting
  // from the specified offset.