Return of $x+x^{2}y+z^2+t^3=0$

We develop techniques for computing the AK invariant of domains with arbitrary characteristic. As an example, we show that for any field $K$ the ring $K[X,Y,Z,T]\mathrm{/}(X+X^{2}Y+Z^2+T^3)$ is not isomorphic to a polynomial ring over $K$.