code: func (s *SubReq) validate() error {
  	// Check the length of the Topic Filter.
  	l := len(s.TopicFilter)
  
  	if l == 0 {
  		return ErrNoTopicFilter
  	}
  
  	if l > maxStringsLen {
  		return ErrTopicFilterExceedsMaxStringsLen
  	}
  
  	// Check the QoS.
  	if !mqtt.ValidQoS(s.QoS) {
  		return ErrInvalidQoS
  	}
  
  	return nil
  }
docs: // validate validates the subscription request.