Biconditional introduction
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Biconditional introduction is the inference that, if B follows from A, and A follows from B, then A if and only if B.
For example: if I'm breathing, then I'm alive; also, if I'm alive, then I'm breathing. Therefore, I'm breathing if and only if I'm alive.
Formally:
( A → B ) ( B → A ) ∴ ( A ↔ B )