Theorem efal ≪ | index | src | ≫ 

theorem efal (a: wff): $ F. -> a $;
StepHypRefExpression
1 absurd 
~F. -> F. -> a
2 notfal 
~F.
3 1, 2 ax_mp 
F. -> a

Axiom use

axs_prop_calc (ax_1, ax_2, ax_3, ax_mp, itru)