Theorem 2th

Two truths are equivalent. (Contributed by NM, 18-Aug-1993.)

Hypotheses
2th.1
\( \vdash \varphi \)
2th.2
\( \vdash \psi \)
Conclusion
\( \vdash \varphi \leftrightarrow \psi \)
Proof
2th.2
l1
a1i _
2th.1
a1i _
impbii l1 _