Theorem sbeq2i ≪ | index | src | ≫

theorem sbeq2i {x: nat} (a: nat x) (b c: wff x):
  $ b <-> c $ >
  $ [a / x] b <-> [a / x] c $;

Step	Hyp	Ref	Expression
1		hyp h	b <-> c
2	1	a1i	T. -> (b <-> c)
3	2	sbeq2d	T. -> ([a / x] b <-> [a / x] c)
4	3	trud	[a / x] b <-> [a / x] c

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)