Theorem rp-frege25

Description: Closed form for a1dd . Alternate route to Proposition 25 of Frege1879 p. 42. (Contributed by RP, 24-Dec-2019)

	Ref	Expression
Assertion	rp-frege25	|- ( ( ph -> ( ps -> ch ) ) -> ( ph -> ( ps -> ( th -> ch ) ) ) )

Proof

Step	Hyp	Ref	Expression
1		rp-frege24	|- ( ( ps -> ch ) -> ( ps -> ( th -> ch ) ) )
2		frege5	|- ( ( ( ps -> ch ) -> ( ps -> ( th -> ch ) ) ) -> ( ( ph -> ( ps -> ch ) ) -> ( ph -> ( ps -> ( th -> ch ) ) ) ) )
3	1 2	ax-mp	|- ( ( ph -> ( ps -> ch ) ) -> ( ph -> ( ps -> ( th -> ch ) ) ) )