code: func (up *Unsuback) Encode(dst []byte) (int, error) {
  	return identifiedEncode(dst, up.ID, UNSUBACK)
  }
docs: // Encode writes the packet bytes into the byte slice from the argument. It
  // returns the number of bytes encoded and whether there's any errors along
  // the way. If there is an error, the byte slice should be considered invalid.