code: def flatten(*caches):
      
      return list(OrderedDict.fromkeys(e for c in caches for e in c))
docs: Flatten a nested list of cache entries
  
      Parameters
      ----------
      *caches : `list`
          One or more lists of file paths
          (`str` or :class:`~lal.utils.CacheEntry`).
  
      Returns
      -------
      flat : `list`
          A flat `list` containing the unique set of entries across
          each input.