Biconditional Introduction/Proof Rule/Tableau Form

Proof Rule
Let $\phi \implies \psi$ and $\psi \implies \phi$ be two conditional statements involving the two well-formed formulas $\phi$ and $\psi$ in a tableau proof.

Biconditional Introduction is invoked for $\phi$ and $\psi$ in the following manner:

Also denoted as
Sources which refer to this rule as the conditional-biconditional rule may as a consequence give the abbreviation $\mathrm {CB}$.