Generate replies.

        Parameters
        ----------
        max_length : `int` or `None`
            Maximum sentence length.
        state_size : `int`
            State size.
        reply_to : `str`
            Input string.
        dataset: `str`
            Dataset key prefix.

        Returns
        -------
        `generator` of `str`
            Generated texts.

def generate_replies(self, max_length, state_size, reply_to, dataset):
        """Generate replies.

        Parameters
        ----------
        max_length : `int` or `None`
            Maximum sentence length.
        state_size : `int`
            State size.
        reply_to : `str`
            Input string.
        dataset: `str`
            Dataset key prefix.

        Returns
        -------
        `generator` of `str`
            Generated texts.
        """
        state_sets = self.get_reply_states(
            reply_to,
            dataset + state_size_dataset(state_size)
        )

        if not state_sets:
            yield from self.generate_cont(max_length, state_size,
                                          None, False, dataset)
            return

        random.shuffle(state_sets)

        generate = lambda state, backward: self.generate(
            state_size, state,
            dataset, backward
        )

        for states in cycle(state_sets):
            state = random.choice(states)
            parts = chain(
                reversed(list(generate(state, True))),
                (state,),
                generate(state, False)
            )
            parts = islice(parts, 0, max_length)
            yield self.format(parts)