Setting a frame id

edit

I have a problem in setting a world frame id. I did as below, but it doesn't work. It that right what I did and where should it actually be set? I've also tried with and without (/), but, it makes no difference at the moment. The error that I suspect is, in the fixed_frame in rviz I couldn't tick for it. What more I have the following warning:

[ WARN] [1329404542.223645698]: Warning: Frame id /world does not exist! Frames (3): Frame /laser exists with parent /base_link.
Frame /base_link exists with parent NO_PARENT.

Code snippet:

    void AssemblerClient::useAssembler(const sensor_msgs::LaserScanConstPtr& scan_ptr)
    {
      ROS_INFO("Assembler service is being used");

      ros::service::waitForService("assemble_scans");
      ros::ServiceClient client = n.serviceClient<AssembleScans>("assemble_scans");
      AssembleScans srv;
      srv.request.begin = ros::Time(0,0);
      srv.request.end   = ros::Time::now();
      if (client.call(srv))
      {
        ROS_INFO("Got cloud with %u points\n", srv.response.cloud.points.size());

        gathered_cloud = srv.response.cloud;
        gathered_cloud.header.frame_id = "/world";

        gathered_cloud_publisher_.publish(gathered_cloud);
      }
        else
        ROS_INFO("Service call failed\n");
    }

Thanks in advance.