Theorem eqscomb ≪ | index | src | ≫ 

theorem eqscomb (A B: set): $ A == B <-> B == A $;
StepHypRefExpression
1 eqscom 
A == B -> B == A
2 eqscom 
B == A -> A == B
3 1, 2 ibii 
A == B <-> B == A

Axiom use

axs_prop_calc (ax_1, ax_2, ax_3, ax_mp), axs_pred_calc (ax_gen, ax_4)