Theorem biim1 | index | src |

theorem biim1 (a b: wff): $ a -> (a -> b <-> b) $;
StepHypRefExpression
1 biim1a
(~a -> b) -> (a -> b <-> b)
2 absurdr
a -> ~a -> b
3 1, 2 syl
a -> (a -> b <-> b)

Axiom use

axs_prop_calc (ax_1, ax_2, ax_3, ax_mp)