Theorem notnot ≪ | index | src | ≫ 

theorem notnot (a: wff): $ a <-> ~~a $;
StepHypRefExpression
1 notnot1 
a -> ~~a
2 dne 
~~a -> a
3 1, 2 ibii 
a <-> ~~a

Axiom use

axs_prop_calc (ax_1, ax_2, ax_3, ax_mp)