Theorem con3d ≪ | index | src | ≫ 

theorem con3d (a b c: wff): $ a -> b -> c $ > $ a -> ~c -> ~b $;
StepHypRefExpression
1 con3 
(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)