The xxz derivative

The 45$^\circ$ diffraction equation differs from the 15$^\circ$ equation by the inclusion of a $\partial^3 / \partial x^2 \partial z$ -derivative. Luckily this derivative fits on the six-point differencing star

-1

2

1

-2

So other than modifying the six coefficients on the star, it adds nothing to the computational cost.