A list of states example -
        [('x1', 'easy'), ('x2', 'hard')]

        Returns
        -------
        True, if arg is a list of states else False.

def is_list_of_states(self, arg):
        """
        A list of states example -
        [('x1', 'easy'), ('x2', 'hard')]

        Returns
        -------
        True, if arg is a list of states else False.

        """
        return isinstance(arg, list) and all(isinstance(i, tuple) for i in arg)