Theorem sbset ≪ | index | src | ≫ 

theorem sbset {x: nat} (a: nat) (A: set x) (B: set):
  $ A. x (x = a -> A == B) -> S[a / x] A == B $;
StepHypRefExpression
1 nfsv 
FS/ x B
2 1 sbseht 
A. x (x = a -> A == B) -> S[a / x] A == 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)