Theorem cbvexeqsetf 3493

Description: The expression ∃𝑥𝑥 = 𝐴 means "𝐴 is a set" even if 𝐴 contains 𝑥 as a bound variable. This lemma helps minimizing axiom or df-clab 2713 usage in some cases. Extracted from the proof of issetft 3494. (Contributed by Wolf Lammen, 30-Jul-2025.)

Assertion

Ref	Expression
cbvexeqsetf	⊢ (Ⅎ𝑥𝐴 → (∃𝑥 𝑥 = 𝐴 ↔ ∃𝑦 𝑦 = 𝐴))

Distinct variable groups:   𝑦,𝐴   𝑥,𝑦

Allowed substitution hint:   𝐴(𝑥)

Proof of Theorem cbvexeqsetf

Step	Hyp	Ref	Expression
1		nfnfc1 2906	. . . 4 ⊢ Ⅎ𝑥Ⅎ𝑥𝐴
2		nfv 1912	. . . 4 ⊢ Ⅎ𝑦Ⅎ𝑥𝐴
3		nfvd 1913	. . . 4 ⊢ (Ⅎ𝑥𝐴 → Ⅎ𝑦 ¬ 𝑥 = 𝐴)
4		nfcvd 2904	. . . . . 6 ⊢ (Ⅎ𝑥𝐴 → Ⅎ𝑥𝑦)
5		id 22	. . . . . 6 ⊢ (Ⅎ𝑥𝐴 → Ⅎ𝑥𝐴)
6	4, 5	nfeqd 2914	. . . . 5 ⊢ (Ⅎ𝑥𝐴 → Ⅎ𝑥 𝑦 = 𝐴)
7	6	nfnd 1856	. . . 4 ⊢ (Ⅎ𝑥𝐴 → Ⅎ𝑥 ¬ 𝑦 = 𝐴)
8		eqeq1 2739	. . . . . 6 ⊢ (𝑥 = 𝑦 → (𝑥 = 𝐴 ↔ 𝑦 = 𝐴))
9	8	notbid 318	. . . . 5 ⊢ (𝑥 = 𝑦 → (¬ 𝑥 = 𝐴 ↔ ¬ 𝑦 = 𝐴))
10	9	a1i 11	. . . 4 ⊢ (Ⅎ𝑥𝐴 → (𝑥 = 𝑦 → (¬ 𝑥 = 𝐴 ↔ ¬ 𝑦 = 𝐴)))
11	1, 2, 3, 7, 10	cbv2w 2338	. . 3 ⊢ (Ⅎ𝑥𝐴 → (∀𝑥 ¬ 𝑥 = 𝐴 ↔ ∀𝑦 ¬ 𝑦 = 𝐴))
12		alnex 1778	. . 3 ⊢ (∀𝑥 ¬ 𝑥 = 𝐴 ↔ ¬ ∃𝑥 𝑥 = 𝐴)
13		alnex 1778	. . 3 ⊢ (∀𝑦 ¬ 𝑦 = 𝐴 ↔ ¬ ∃𝑦 𝑦 = 𝐴)
14	11, 12, 13	3bitr3g 313	. 2 ⊢ (Ⅎ𝑥𝐴 → (¬ ∃𝑥 𝑥 = 𝐴 ↔ ¬ ∃𝑦 𝑦 = 𝐴))
15	14	con4bid 317	1 ⊢ (Ⅎ𝑥𝐴 → (∃𝑥 𝑥 = 𝐴 ↔ ∃𝑦 𝑦 = 𝐴))

Syntax hints:  ¬ wn 3   → wi 4   ↔ wb 206  ∀wal 1535   = wceq 1537  ∃wex 1776  Ⅎwnfc 2888

This theorem was proved from axioms:  ax-mp 5  ax-1 6  ax-2 7  ax-3 8  ax-gen 1792  ax-4 1806  ax-5 1908  ax-6 1965  ax-7 2005  ax-9 2116  ax-10 2139  ax-11 2155  ax-12 2175  ax-ext 2706

This theorem depends on definitions:  df-bi 207  df-an 396  df-or 848  df-ex 1777  df-nf 1781  df-cleq 2727  df-nfc 2890

This theorem is referenced by:  issetft  3494  spcimgft  3546