protected function addToRequest( $request_key, $key, $new_items ) {
		$this->validateStore();

		$new_items = MixedType::toArray( $new_items );
		$items = MixedType::toArray( $this->get( $key, [] ) );
		$this->flashed[ $request_key ][ $key ] = array_merge( $items, $new_items );
	}