def _non_linear_cspline(wcs_dict):
    """Returns a cubic spline model of the wavelength solution.

    This function should extract certain parameters from the `wcs_dict`
    parameter and construct a mathematical model that makes the conversion from
    pixel to wavelength. All the necessary information is already contained in
    the dictionary so the only work to be done is to make the instantiation of
    the appropriate subclass of `~astropy.modeling.Model`.

    Not implemented

     Parameters
    ----------

        wcs_dict : dict
            Contains all the WCS information decoded from an IRAF fits header.

    Raises
    ------

        NotImplementedError
    """
    raise NotImplementedError('Cubic spline is not implemented')
