Theorem ex2eq ≪ | index | src | ≫ 

theorem ex2eq (_R1 _R2: set): $ _R1 == _R2 -> ex2 _R1 == ex2 _R2 $;
StepHypRefExpression
1 id 
_R1 == _R2 -> _R1 == _R2
2 1 ex2eqd 
_R1 == _R2 -> ex2 _R1 == ex2 _R2

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)