code: public <T extends SubSystem> void addSystem(Class<T> systemApi, T system)
    {
      SubSystem oldSystem
        = _systemMap.putIfAbsent(systemApi, system);
  
      if (oldSystem != null) {
        throw new IllegalStateException(L.l("duplicate system '{0}' is not allowed because another system with that class is already registered '{1}'",
                                            system, oldSystem));
      }
  
      _pendingStart.add(system);
  
      if (_lifecycle.isActive()) {
        startSystems();
      }
    }
docs: Adds a new system.