Get a random link.

        Parameters
        ----------
        dataset : `object`
            Dataset from `self.get_dataset()`.
        state : `object`
            Link source.
        backward : `bool`, optional
            Link direction.

        Raises
        ------
        ValueError
            If link count is invalid.

        Returns
        -------
        (`str` or `None`, `object` or `None`)
            Link value and next state.

def random_link(self, dataset, state, backward=False):
        """Get a random link.

        Parameters
        ----------
        dataset : `object`
            Dataset from `self.get_dataset()`.
        state : `object`
            Link source.
        backward : `bool`, optional
            Link direction.

        Raises
        ------
        ValueError
            If link count is invalid.

        Returns
        -------
        (`str` or `None`, `object` or `None`)
            Link value and next state.
        """
        links = self.get_links(dataset, state, backward)
        if not links:
            return None, None
        x = randint(0, sum(link[0] for link in links) - 1)
        for link in links:
            count = link[0]
            if x < count:
                return link[1], self.follow_link(link, state, backward)
            x -= count
        raise RuntimeError('invalid link sum')