Theorem nfeqi ≪ | index | src | ≫

theorem nfeqi {x: nat} (a b: wff x): $ a <-> b $ > $ (F/ x a) <-> (F/ x b) $;

Step	Hyp	Ref	Expression
1		hyp h	a <-> b
2	1	a1i	T. -> (a <-> b)
3	2	nfeqd	T. -> ((F/ x a) <-> (F/ x b))
4	3	trud	(F/ x a) <-> (F/ x b)

Axiom use

axs_prop_calc (ax_1, ax_2, ax_3, ax_mp, itru), axs_pred_calc (ax_gen, ax_4, ax_5)