Theorem noteqd ≪ | index | src | ≫ 

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

Axiom use

axs_prop_calc (ax_1, ax_2, ax_3, ax_mp)