Theorem biidd

Description: Principle of identity with antecedent. (Contributed by NM, 25-Nov-1995)

	Ref	Expression
Assertion	biidd	$$⊢\phi \to \left(\psi \leftrightarrow \psi \right)$$

Proof

Step	Hyp	Ref	Expression
1		biid	$$⊢\psi \leftrightarrow \psi$$
2	1	a1i	$$⊢\phi \to \left(\psi \leftrightarrow \psi \right)$$