Give proofs for the theorems of $\mathscr{F}^\omega$ stated below. Use appropriate analogues of the derived rules of inference of $$\$ 21$$.
$\vdash x_\tau=y_\tau \supset . S_{x_\tau}^{x_\tau} A \supset S_{x_\tau}^{z_\tau} A$ provided $x_\tau$ and $y_\tau$ are free for $z_\tau$ in $A$.