docs: def extant_file(file):
      """
      'Type' for argparse - checks that file exists but does not open.
      """
      if not os.path.exists(file):
          # Argparse uses the ArgumentTypeError to give a rejection message like:
          # error: argument input: file does not exist
          raise argparse.ArgumentTypeError("{0} does not exist".format(file))
      return file