Step 1:
Understand the theorem to be proved:
$$\vdash x_{(\mathrm{f})}=y_{(\mathrm{f})} \supset \forall z_q \cdot y z \supset x z$$
This theorem states that if two functionally subscripted variables $x_{(\mathrm{f})}$ and $y_{(\mathrm{f})}$ are equal, then for any
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