\item \self Given the formula $$\phi = \forall x \exists z \; \big( \lnot P(x) \lor Q(y, f(z))\big) \imp \big(\lnot \exists x \; P(y) \land Q(f(x), z)\big).$$ 

\begin{enumerate}
    \item  Compute $\phi[f(y)/x]$.
    \item  Compute $\phi[f(x)/y]$.
    \item  Compute $\phi[k/z]$.
    \item  Compute $\phi[x/z]$.
\end{enumerate}