'Vector sum.
The representation is converted to cartesian, the sums of the x, y,
and z components are calculated, and the result is converted to a
`~astropy.coordinates.SphericalRepresentation`.
Refer to `~numpy.sum` for full documentation of the arguments, noting
that ``axis`` is the entry in the ``shape`` of the representation, and
that the ``out`` argument cannot be used.'

def sum(self, *args, **kwargs):

    self._raise_if_has_differentials(u'sum')
    return self._dimensional_representation.from_cartesian(self.to_cartesian().sum(*args, **kwargs))
