Theorem con1d ≪ | index | src | ≫ 

theorem con1d (a b c: wff): $ a -> ~b -> c $ > $ a -> ~c -> b $;
StepHypRefExpression
1 con1 
(~b -> c) -> ~c -> b
2 hyp h 
a -> ~b -> c
3 1, 2 syl 
a -> ~c -> b

Axiom use

axs_prop_calc (ax_1, ax_2, ax_3, ax_mp)