Theorem pm4.56

Description: Theorem *4.56 of WhiteheadRussell p. 120. (Contributed by NM, 3-Jan-2005)

	Ref	Expression
Assertion	pm4.56	$$⊢\left(\neg \phi \wedge \neg \psi \right)\leftrightarrow \neg \left(\phi \vee \psi \right)$$

Proof

Step	Hyp	Ref	Expression
1		ioran	$$⊢\neg \left(\phi \vee \psi \right)\leftrightarrow \left(\neg \phi \wedge \neg \psi \right)$$
2	1	bicomi	$$⊢\left(\neg \phi \wedge \neg \psi \right)\leftrightarrow \neg \left(\phi \vee \psi \right)$$