docs: def purge():
      """
      Deletes all the cached files since the last call to reset_used that have not been used.
  
      """
      all_hashes = read_all()
      used_hashes = read_used()
      for kind, hashes in used_hashes.items():
          hashes = set(hashes)
          to_remove = set(all_hashes[kind]).difference(hashes)
          delete_from_directory_by_hashes(kind, to_remove)
  
      reset_used()
      write_out()