Theorem elun | index | src |

theorem elun (A B: set) (a: nat): $ a e. A u. B <-> a e. A \/ a e. B $;
StepHypRefExpression
1 eleq1
x = a -> (x e. A <-> a e. A)
2 eleq1
x = a -> (x e. B <-> a e. B)
3 1, 2 oreqd
x = a -> (x e. A \/ x e. B <-> a e. A \/ a e. B)
4 3 elabe
a e. {x | x e. A \/ x e. B} <-> a e. A \/ a e. B
5 4 conv Union
a e. A u. B <-> a e. A \/ a e. 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)