'Vector sum.
Returns a new CartesianRepresentation instance with the sums of the
x, y, and z components.
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._apply(u'sum', *args, **kwargs)
