code: def preload_top(meta):
      
      top_fns = set(meta['top_fn'])
      if len(top_fns) != 1:
          raise ValueError("More than one topology is used in this project!")
      return md.load_topology(top_fns.pop())
docs: Load one topology file into memory.
  
      This function checks to make sure there's only one topology file
      in play. When sampling frames, you have to have all the same
      topology to concatenate.
  
      Parameters
      ----------
      meta : pd.DataFrame
          The DataFrame of metadata with a column named 'top_fn'
  
      Returns
      -------
      top : md.Topology
          The one topology file that can be used for all trajectories.