Does it make sense for ack to hold true on the initial state of path $\pi$? If we consider another path $\pi_b,ドル and $\pi$ is a suffix of $\pi_b,ドル so that $\pi = \pi_b^k$ for some $k,ドル and $\pi_b^{k-1} \models \req,ドル then it would make sense for $\pi^0 \models \ack$ to hold true.