Theorem eqcomb ≪ | index | src | ≫ 

theorem eqcomb (a b: nat): $ a = b <-> b = a $;
StepHypRefExpression
1 eqcom 
a = b -> b = a
2 eqcom 
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, ax_5, ax_6, ax_7)