Theorem bior2 ≪ | index | src | ≫ 

theorem bior2 (a b: wff): $ ~b -> (a \/ b <-> a) $;
StepHypRefExpression
1 bior2a 
(b -> a) -> (a \/ b <-> a)
2 absurd 
~b -> b -> a
3 1, 2 syl 
~b -> (a \/ b <-> a)

Axiom use

axs_prop_calc (ax_1, ax_2, ax_3, ax_mp)