Metamath Proof Explorer


Theorem pm2.521

Description: Theorem *2.521 of WhiteheadRussell p. 107. Instance of pm2.521g and of pm2.521g2 . (Contributed by NM, 3-Jan-2005)

Ref Expression
Assertion pm2.521 ¬ φ ψ ψ φ

Proof

Step Hyp Ref Expression
1 pm2.521g ¬ φ ψ ψ φ