Theorem 2albii

Inference adding two universal quantifiers to both sides of an equivalence. (Contributed by NM, 9-Mar-1997.)

Hypothesis
albii.1
\( \vdash \varphi \leftrightarrow \psi \)
Conclusion
\( \vdash \forall x \, \forall y \, \varphi \leftrightarrow \forall x \, \forall y \, \psi \)
Proof
albii.1
albii _
albii _