Theorem imbi2i

Introduce an antecedent to both sides of a logical equivalence. This and the next three rules are useful for building up wff's around a definition, in order to make use of the definition. (Contributed by NM, 3-Jan-1993.) (Proof shortened by Wolf Lammen, 6-Feb-2013.)

Hypothesis
imbi2i.1
\( \vdash \varphi \leftrightarrow \psi \)
Conclusion
\( \vdash \chi \rightarrow \varphi \leftrightarrow \chi \rightarrow \psi \)
Proof
imbi2i.1
a1i _
pm5.74i _