Theorem dne ≪ | index | src | ≫ 

theorem dne (a: wff): $ ~~a -> a $;
StepHypRefExpression
1 contra 
(~a -> a) -> a
2 absurd 
~~a -> ~a -> a
3 1, 2 syl 
~~a -> a

Axiom use

axs_prop_calc (ax_1, ax_2, ax_3, ax_mp)