Theorem cbvexs ≪ | index | src | ≫

theorem cbvexs {x y: nat} (p: wff x): $ E. x p <-> E. y [y / x] p $;

Step	Hyp	Ref	Expression
1		nfv	F/ y p
2		nfsb1	F/ x [y / x] p
3		sbq	x = y -> (p <-> [y / x] p)
4	1, 2, 3	cbvexh	E. x p <-> E. y [y / x] p

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)