docs: public int getLengthPrefix() {
      // Message ID
      int length = 1;
      // Number of rules
      length += 4;
      if (this.rules != null) {
        for (SubscriptionRequestRule rule : this.rules) {
          // Physical Layer
          ++length;
          // Transmitters
          length += 4 + rule.getNumTransmitters()
              * Transmitter.TRANSMITTER_ID_SIZE * 2;
          // Update interval
          length += 8;
        }
  
      }
  
      return length;
    }