Theorem drex2

Formula-building lemma for use with the Distinctor Reduction Theorem. Part of Theorem 9.4 of [Megill] p. 448 (p. 16 of preprint). Usage of this theorem is discouraged because it depends on [ax-13]. Usage of exbidv is preferred, which requires fewer axioms. (Contributed by NM, 27-Feb-2005.) (New usage is discouraged.)

Hypothesis
dral1.1
\( \vdash \forall x \, x = y \rightarrow (\varphi \leftrightarrow \psi) \)
Conclusion
\( \vdash \forall x \, x = y \rightarrow (\exists z \, \varphi \leftrightarrow \exists z \, \psi) \)
Proof
l0
nfae
dral1.1
exbid l0 _