Theorem ssun1 | index | src |

theorem ssun1 (A B: set): $ A C_ A u. B $;
StepHypRefExpression
1 elun
x e. A u. B <-> x e. A \/ x e. B
2 orl
x e. A -> x e. A \/ x e. B
3 1, 2 sylibr
x e. A -> x e. A u. B
4 3 ax_gen
A. x (x e. A -> x e. A u. B)
5 4 conv subset
A C_ 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)