Theorem bi1i ≪ | index | src | ≫ 

theorem bi1i (a b: wff): $ a <-> b $ > $ a -> b $;
StepHypRefExpression
1 bi1 
(a <-> b) -> a -> b
2 hyp h 
a <-> b
3 1, 2 ax_mp 
a -> b

Axiom use

axs_prop_calc (ax_1, ax_2, ax_3, ax_mp)