Biconditional Introduction/Proof Rule

Proof Rule
The Rule of Biconditional Introduction is a valid deduction sequent in propositional logic.

As a proof rule it is expressed in the form:
 * If we can conclude both $\phi \implies \psi$ and $\psi \implies \phi$, then we may infer $\phi \iff \psi$.

It can be written:
 * $\ds { {\phi \implies \psi \qquad \psi \implies \phi} \over \phi \iff \psi} \iff_i$

Thus it is used to introduce the biconditional operator into a sequent.