b' < 'a']

        >>> ser.sort_values()
        2   c
        1   b
        0   a
        3   a
        dtype: category
        Categories (3, object): ['c' < 'b' < 'a']

        For :class:`pandas.CategoricalIndex`:

        >>> ci = pd.CategoricalIndex(['a', 'b', 'c', 'a'])
        >>> ci
        CategoricalIndex(['a', 'b', 'c', 'a'], categories=['a', 'b', 'c'],
                         ordered=False, dtype='category')
        >>> ci.reorder_categories(['c', 'b', 'a'], ordered=True)
        CategoricalIndex(['a', 'b', 'c', 'a'], categories=['c', 'b', 'a'],
                         ordered=True, dtype='category')
        z=items in new_categories are not the same as in old categoriesr‡