code: func (p *PUBREL) setFixedHeader() {
  	// Append the first byte to the fixed header.
  	p.fixedHeader = append(p.fixedHeader, TypePUBREL<<4|0x02)
  
  	// Append the Remaining Length to the fixed header.
  	p.appendRemainingLength()
  }
docs: // setFixedHeader sets the fixed header to the Packet.