code: def save_trajs(trajs, fn, meta, key_to_path=None):
      
      if key_to_path is None:
          key_to_path = default_key_to_path
  
      validate_keys(meta.index, key_to_path)
      backup(fn)
      os.mkdir(fn)
      for k in meta.index:
          v = trajs[k]
          npy_fn = os.path.join(fn, key_to_path(k))
          os.makedirs(os.path.dirname(npy_fn), exist_ok=True)
          np.save(npy_fn, v)
docs: Save trajectory-like data
  
      Data is stored in individual numpy binary files in the
      directory given by ``fn``.
  
      This method will automatically back up existing files named ``fn``.
  
      Parameters
      ----------
      trajs : dict of (key, np.ndarray)
          Dictionary of trajectory-like ndarray's keyed on ``meta.index``
          values.
      fn : str
          Where to save the data. This will be a directory containing
          one file per trajectory
      meta : pd.DataFrame
          The DataFrame of metadata