docs: function Selection(serviceRegistry, selectionServiceId) {
  		if (!selectionServiceId) {
  			selectionServiceId = "orion.page.selection"; //$NON-NLS-0$
  		}
  		
  		this._serviceRegistry = serviceRegistry;
  		EventTarget.attach(this);
  		this._serviceRegistration = serviceRegistry.registerService(selectionServiceId, this);
  		this._selections = null;  // so we can quickly recognize the "empty" case without comparing arrays.
  	}