code: protected static <E extends SubSystemBase> SystemManager
      preCreate(Class<E> serviceClass)
    {
      SystemManager system = SystemManager.getCurrent();
      if (system == null)
        throw new IllegalStateException(L.l("{0} must be created before {1}",
                                            SystemManager.class.getSimpleName(),
                                            serviceClass.getSimpleName()));
      
      if (system.getSystem(serviceClass) != null)
        throw new IllegalStateException(L.l("{0} was previously created",
                                            serviceClass.getSimpleName()));
      
      return system;
    }
docs: convenience method for subclass's create methods