ts`` list of elements.

        Example::

            numlist = Word(nums)[...]
            print(numlist.parse_string("0 123 321")) # -> ['0', '123', '321']

            # use a parse action to compute the sum of the parsed integers, and add it to the end
            def append_sum(tokens):
                tokens.append(sum(map(int, tokens)))
            numlist.add_parse_action(append_sum)
            print(numlist.parse_string("0 123 321")) # -> ['0', '123', '321', 444]
        N)