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Metamath Proof Explorer

Theorem pm4.57

Description: Theorem *4.57 of WhiteheadRussell p. 120. (Contributed by NM, 3-Jan-2005)

Ref Expression
Assertion pm4.57 ⊢ ¬ ¬ φ ∧ ¬ ψ ↔ φ ∨ ψ

Proof

Step Hyp Ref Expression
1 oran ⊢ φ ∨ ψ ↔ ¬ ¬ φ ∧ ¬ ψ
2 1 bicomi ⊢ ¬ ¬ φ ∧ ¬ ψ ↔ φ ∨ ψ