code: def _chk_docopt_exit(self, args, exp_letters):
          
          if args is None:
              args = sys.argv[1:]
          keys_all = self.exp_keys.union(self.exp_elems)
          if exp_letters:
              keys_all |= exp_letters
          unknown_args = self._chk_docunknown(args, keys_all)
          if unknown_args:
              raise RuntimeError("{USAGE}\n    **FATAL: UNKNOWN ARGS: {UNK}".format(
                  USAGE=self.doc, UNK=" ".join(unknown_args)))
docs: Check if docopt exit was for an unknown argument.