Theorem pm4.25

Description: Theorem *4.25 of WhiteheadRussell p. 117. (Contributed by NM, 3-Jan-2005)

	Ref	Expression
Assertion	pm4.25	$$⊢\phi \leftrightarrow \left(\phi \vee \phi \right)$$

Proof

Step	Hyp	Ref	Expression
1		oridm	$$⊢\left(\phi \vee \phi \right)\leftrightarrow \phi$$
2	1	bicomi	$$⊢\phi \leftrightarrow \left(\phi \vee \phi \right)$$