code: def preload_tops(meta):
      
      top_fns = set(meta['top_fn'])
      tops = {}
      for tfn in top_fns:
          tops[tfn] = md.load_topology(tfn)
      return tops
docs: Load all topology files into memory.
  
      This might save some performance compared to re-parsing the topology
      file for each trajectory you try to load in. Typically, you have far
      fewer (possibly 1) topologies than trajectories
  
      Parameters
      ----------
      meta : pd.DataFrame
          The DataFrame of metadata with a column named 'top_fn'
  
      Returns
      -------
      tops : dict
          Dictionary of ``md.Topology`` objects, keyed by "top_fn"
          values.