Theorem con3 ≪ | index | src | ≫

theorem con3 (a b: wff): $ (a -> b) -> ~b -> ~a $;

Step	Hyp	Ref	Expression
1		con2	(a -> ~~b) -> ~b -> ~a
2		notnot1	b -> ~~b
3	2	imim2i	(a -> b) -> a -> ~~b
4	1, 3	syl	(a -> b) -> ~b -> ~a

Axiom use

axs_prop_calc (ax_1, ax_2, ax_3, ax_mp)