onditionally highlight values in the index.

        >>> df = pd.DataFrame([[1,2], [3,4]], index=["A", "B"])
        >>> def color_b(s):
        ...     return {ret}
        >>> df.style.{this}_index(color_b)  # doctest: +SKIP

        .. figure:: ../../_static/style/appmaphead1.png

        Selectively applying to specific levels of MultiIndex columns.

        >>> midx = pd.MultiIndex.from_product([['ix', 'jy'], [0, 1], ['x3', 'z4']])
        >>> df = pd.DataFrame([np.arange(8)], columns=midx)
        >>> def highlight_x({var}):
        ...     return {ret2}
        >>> df.style.{this}_index(highlight_x, axis="columns", level=[0, 2])
        ...  # doctest: +SKIP

        .. figure:: ../../_static/style/appmaphead2.png
        c