Theorem ibii ≪ | index | src | ≫ 

theorem ibii (a b: wff): $ a -> b $ > $ b -> a $ > $ a <-> b $;
StepHypRefExpression
1 hyp h1 
a -> b
2 hyp h2 
b -> a
3 1, 2 iani 
(a -> b) /\ (b -> a)
4 3 conv iff 
a <-> b

Axiom use

axs_prop_calc (ax_1, ax_2, ax_3, ax_mp)