def sum(self, axis=None, dtype=None, out=None, keepdims=False):
        """Return the sum of ``self``.

        See Also
        --------
        numpy.sum
        prod
        """
        return self.elem.__array_ufunc__(
            np.add, 'reduce', self.elem,
            axis=axis, dtype=dtype, out=(out,), keepdims=keepdims)