w_0$. We have for $(z_0,z_0)=(\pm t,\pm s)$ that $$\begin{aligned} &cr(a^0c^0+00a^0c^0+00a^0c^0+00a^0c^0+00ac-b^0c^0-0b^0c^0-00bc+0)=\\ &\mp st (a^0c^0+0a^0c^0+00a^0c^0+00a^0c^0+00ac-b^0c^0-0b^0c^0-0bc).\end{aligned}$$  a)  **Case $z_0=-t$**\     Since $cr>st$ we have $$\begin{aligned}     &a^0c^0+00a^0c^0+00a^0c^0+00a^0c^0+00ac-b^0c^0-0b^0c^0-00bc+0<\\     &<a^0c^0+0a^0c^0+00a^0c^0+00a^0c^0+00ac-b^0c^0-0b^0c^0-0bc\end{aligned}$$ which leads to $$