def get_diff_str(self, element, length):
        '''get_diff_str

        High-level api: Produce a string that indicates the difference between
        two models.

        Parameters
        ----------

        element : `Element`
            A node in model tree.

        length : `int`
            String length that has been consumed.

        Returns
        -------

        str
            A string that indicates the difference between two models.
        '''

        spaces = ' '*(self.get_width(element) - length)
        return spaces + element.get('diff')