Theorem bi2i ≪ | index | src | ≫ 

theorem bi2i (a b: wff): $ a <-> b $ > $ b -> a $;
StepHypRefExpression
1 bi2 
(a <-> b) -> b -> a
2 hyp h 
a <-> b
3 1, 2 ax_mp 
b -> a

Axiom use

axs_prop_calc (ax_1, ax_2, ax_3, ax_mp)