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Metamath Proof Explorer

Theorem pm2.4

Description: Theorem *2.4 of WhiteheadRussell p. 106. (Contributed by NM, 3-Jan-2005)

Ref Expression
Assertion pm2.4 ⊢ φ ∨ φ ∨ ψ → φ ∨ ψ

Proof

Step Hyp Ref Expression
1 orc ⊢ φ → φ ∨ ψ
2 id ⊢ φ ∨ ψ → φ ∨ ψ
3 1 2 jaoi ⊢ φ ∨ φ ∨ ψ → φ ∨ ψ