Theorem con4d

Description: Deduction associated with con4 . (Contributed by NM, 26-Mar-1995)

	Ref	Expression
Hypothesis	con4d.1	⊢ ( 𝜑 → ( ¬ 𝜓 → ¬ 𝜒 ) )
Assertion	con4d	⊢ ( 𝜑 → ( 𝜒 → 𝜓 ) )

Proof

Step	Hyp	Ref	Expression
1		con4d.1	⊢ ( 𝜑 → ( ¬ 𝜓 → ¬ 𝜒 ) )
2		con4	⊢ ( ( ¬ 𝜓 → ¬ 𝜒 ) → ( 𝜒 → 𝜓 ) )
3	1 2	syl	⊢ ( 𝜑 → ( 𝜒 → 𝜓 ) )