Theorem frege55lem2a

Description: Core proof of Proposition 55 of Frege1879 p. 50. (Contributed by RP, 24-Dec-2019) (Proof modification is discouraged.)

	Ref	Expression
Assertion	frege55lem2a	⊢ ( ( 𝜑 ↔ 𝜓 ) → if- ( 𝜓 , 𝜑 , ¬ 𝜑 ) )

Proof

Step	Hyp	Ref	Expression
1		bicom1	⊢ ( ( 𝜑 ↔ 𝜓 ) → ( 𝜓 ↔ 𝜑 ) )
2		frege54cor0a	⊢ ( ( 𝜓 ↔ 𝜑 ) ↔ if- ( 𝜓 , 𝜑 , ¬ 𝜑 ) )
3	1 2	sylib	⊢ ( ( 𝜑 ↔ 𝜓 ) → if- ( 𝜓 , 𝜑 , ¬ 𝜑 ) )