code: func (p *SUBSCRIBE) setPayload() {
  	// Append each subscription request to the payload.
  	for _, s := range p.SubReqs {
  		// Append the Topic Filter to the payload.
  		p.payload = appendLenStr(p.payload, s.TopicFilter)
  
  		// Append the QoS to the payload.
  		p.payload = append(p.payload, s.QoS)
  	}
  }
docs: // setPayload sets the payload to the Packet.