code: @SuppressWarnings("unchecked")
    public <T extends SubSystem> T
    addSystemIfAbsent(Class<?> systemApi, T system)
    {
      SubSystem oldService
        = _systemMap.putIfAbsent(systemApi, system);
  
      if (oldService != null) {
        return (T) oldService;
      }
  
      _pendingStart.add(system);
  
      if (_lifecycle.isActive()) {
        startSystems();
      }
  
      return null;
    }
docs: Adds a new system.