code: func ComposeVersionNegotiation(destConnID, srcConnID protocol.ConnectionID, versions []protocol.VersionNumber) ([]byte, error) {
  	greasedVersions := protocol.GetGreasedVersions(versions)
  	expectedLen := 1 /* type byte */ + 4 /* version field */ + 1 /* connection ID length field */ + destConnID.Len() + srcConnID.Len() + len(greasedVersions)*4
  	buf := bytes.NewBuffer(make([]byte, 0, expectedLen))
  	r := make([]byte, 1)
  	_, _ = rand.Read(r) // ignore the error here. It is not critical to have perfect random here.
  	buf.WriteByte(r[0] | 0xc0)
  	utils.BigEndian.WriteUint32(buf, 0) // version 0
  	connIDLen, err := encodeConnIDLen(destConnID, srcConnID)
  	if err != nil {
  		return nil, err
  	}
  	buf.WriteByte(connIDLen)
  	buf.Write(destConnID)
  	buf.Write(srcConnID)
  	for _, v := range greasedVersions {
  		utils.BigEndian.WriteUint32(buf, uint32(v))
  	}
  	return buf.Bytes(), nil
  }
docs: // ComposeVersionNegotiation composes a Version Negotiation