Theorem pm2.73

Description: Theorem *2.73 of WhiteheadRussell p. 108. (Contributed by NM, 3-Jan-2005)

	Ref	Expression
Assertion	pm2.73	⊢ ( ( 𝜑 → 𝜓 ) → ( ( ( 𝜑 ∨ 𝜓 ) ∨ 𝜒 ) → ( 𝜓 ∨ 𝜒 ) ) )

Proof

Step	Hyp	Ref	Expression
1		pm2.621	⊢ ( ( 𝜑 → 𝜓 ) → ( ( 𝜑 ∨ 𝜓 ) → 𝜓 ) )
2	1	orim1d	⊢ ( ( 𝜑 → 𝜓 ) → ( ( ( 𝜑 ∨ 𝜓 ) ∨ 𝜒 ) → ( 𝜓 ∨ 𝜒 ) ) )