Disallow states that are not useful to continue simulating.

def _disallow_state(self, state):
        """Disallow states that are not useful to continue simulating."""
        disallow_methods = (self._is_duplicate_board,
                            self._is_impossible_by_count)
        for disallow_method in disallow_methods:
            if disallow_method(state):
                return True
        return False