Theorem pm2.21ddALT

Description: Alternate proof of pm2.21dd . (Contributed by Mario Carneiro, 9-Feb-2017) (Proof modification is discouraged.) (New usage is discouraged.)

	Ref	Expression
Hypotheses	pm2.21ddALT.1	⊢ ( 𝜑 → 𝜓 )
	pm2.21ddALT.2	⊢ ( 𝜑 → ¬ 𝜓 )
Assertion	pm2.21ddALT	⊢ ( 𝜑 → 𝜒 )

Proof

Step	Hyp	Ref	Expression
1		pm2.21ddALT.1	⊢ ( 𝜑 → 𝜓 )
2		pm2.21ddALT.2	⊢ ( 𝜑 → ¬ 𝜓 )
3	2	pm2.21d	⊢ ( 𝜑 → ( 𝜓 → 𝜒 ) )
4	1 3	mpd	⊢ ( 𝜑 → 𝜒 )