Theorem ssun2 | index | src |

theorem ssun2 (A B: set): $ B C_ A u. B $;
StepHypRefExpression
1 sseq2
B u. A == A u. B -> (B C_ B u. A <-> B C_ A u. B)
2 uncom
B u. A == A u. B
3 1, 2 ax_mp
B C_ B u. A <-> B C_ A u. B
4 ssun1
B C_ B u. A
5 3, 4 mpbi
B 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)