'Calculate the resulting wcs.
There is actually no calculation involved but it is a good place to
compare wcs information of both operands. This is currently not working
properly with `~astropy.wcs.WCS` (which is the suggested class for
storing as wcs property) but it will not break it neither.
Parameters
operation : callable
see :meth:`NDArithmeticMixin._arithmetic` parameter description.
By default, the ``operation`` will be ignored.
operand : `NDData` instance or subclass
The second operand wrapped in an instance of the same class as
self.
compare_wcs : callable
see :meth:`NDArithmeticMixin.add` parameter description.
kwds :
Additional parameters given to ``compare_wcs``.
Raises
ValueError
If ``compare_wcs`` returns ``False``.
Returns
result_wcs : any type
The ``wcs`` of the first operand is returned.'

def _arithmetic_wcs(self, operation, operand, compare_wcs, **kwds):

    if (not compare_wcs(self.wcs, operand.wcs, **kwds)):
        raise ValueError(u'WCS    are    not    equal.')
    return self.wcs
