Return the list of nodes at level ``index``,
        in DFS order.

        :param int index: the index
        :rtype: list of :class:`~aeneas.tree.Tree`

        :raises: ValueError if the given ``index`` is not valid

def level_at_index(self, index):
        """
        Return the list of nodes at level ``index``,
        in DFS order.

        :param int index: the index
        :rtype: list of :class:`~aeneas.tree.Tree`

        :raises: ValueError if the given ``index`` is not valid
        """
        if not isinstance(index, int):
            self.log_exc(u"Index is not an integer", None, True, TypeError)
        levels = self.levels
        if (index < 0) or (index >= len(levels)):
            self.log_exc(u"The given level index '%d' is not valid" % (index), None, True, ValueError)
        return self.levels[index]