Theorem idd

Description: Principle of identity id with antecedent. (Contributed by NM, 26-Nov-1995)

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

Proof

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