code: def instance():
          
          if TrajectoryInfoCache._instance is None:
              # if we do not have a configuration director yet, we do not want to store
              if not config.cfg_dir:
                  filename = None
              else:
                  filename = os.path.join(config.cfg_dir, "traj_info.sqlite3")
              TrajectoryInfoCache._instance = TrajectoryInfoCache(filename)
  
          return TrajectoryInfoCache._instance
docs: :returns the TrajectoryInfoCache singleton instance