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Metamath Proof Explorer

Theorem pm2.85

Description: Theorem *2.85 of WhiteheadRussell p. 108. (Contributed by NM, 3-Jan-2005) (Proof shortened by Wolf Lammen, 5-Jan-2013)

Ref Expression
Assertion pm2.85 ⊢ φ ∨ ψ → φ ∨ χ → φ ∨ ψ → χ

Proof

Step Hyp Ref Expression
1 orimdi ⊢ φ ∨ ψ → χ ↔ φ ∨ ψ → φ ∨ χ
2 1 biimpri ⊢ φ ∨ ψ → φ ∨ χ → φ ∨ ψ → χ