Metamath Proof Explorer


Theorem pm4.64

Description: Theorem *4.64 of WhiteheadRussell p. 120. (Contributed by NM, 3-Jan-2005)

Ref Expression
Assertion pm4.64 ¬ φ ψ φ ψ

Proof

Step Hyp Ref Expression
1 df-or φ ψ ¬ φ ψ
2 1 bicomi ¬ φ ψ φ ψ