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Metamath Proof Explorer

Theorem pm4.62

Description: Theorem *4.62 of WhiteheadRussell p. 120. (Contributed by NM, 3-Jan-2005)

Ref Expression
Assertion pm4.62 ⊢ φ → ¬ ψ ↔ ¬ φ ∨ ¬ ψ

Proof

Step Hyp Ref Expression
1 imor ⊢ φ → ¬ ψ ↔ ¬ φ ∨ ¬ ψ