Theorem elun1 ≪ | index | src | ≫ 

theorem elun1 (A B: set) (a: nat): $ a e. A -> a e. A u. B $;
StepHypRefExpression
1 ssel 
A C_ A u. B -> a e. A -> a e. A u. B
2 ssun1 
A C_ A u. B
3 1, 2 ax_mp 
a e. A -> a e. A u. B

Axiom use

axs_prop_calc (ax_1, ax_2, ax_3, ax_mp, itru), axs_pred_calc (ax_gen, ax_4, ax_5, ax_6, ax_7, ax_10, ax_11, ax_12), axs_set (elab, ax_8)