Metamath Proof Explorer


Theorem pm4.56

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

Ref Expression
Assertion pm4.56 ¬φ¬ψ¬φψ

Proof

Step Hyp Ref Expression
1 ioran ¬φψ¬φ¬ψ
2 1 bicomi ¬φ¬ψ¬φψ