fmlafvel - Mathbox for Mario Carneiro

	Mathbox for Mario Carneiro		< Previous Next > Nearby theorems
Mirrors > Home > MPE Home > Th. List > Mathboxes > fmlafvel		Structured version Visualization version GIF version

Theorem fmlafvel 34931

Description: A class is a valid Godel formula of height 𝑁 iff it is the first component of a member of the value of the satisfaction predicate as function over wff codes in the empty model with an empty binary relation at 𝑁. (Contributed by AV, 19-Sep-2023.)

**Assertion**
Ref	Expression
fmlafvel	⊢ (𝑁 ∈ ω → (𝐹 ∈ (Fmla‘𝑁) ↔ ⟨𝐹, ∅⟩ ∈ ((∅ Sat ∅)‘𝑁)))

Proof of Theorem fmlafvel Dummy variables 𝑢 𝑣 𝑥 𝑦 𝑖 𝑗 𝑧 are mutually distinct and distinct from all other variables.

Step	Hyp	Ref	Expression
1		fveq2 6891	. . . . . . 7 ⊢ (𝑥 = ∅ → (Fmla‘𝑥) = (Fmla‘∅))
2	1	eleq2d 2814	. . . . . 6 ⊢ (𝑥 = ∅ → (𝐹 ∈ (Fmla‘𝑥) ↔ 𝐹 ∈ (Fmla‘∅)))
3		fveq2 6891	. . . . . . 7 ⊢ (𝑥 = ∅ → ((∅ Sat ∅)‘𝑥) = ((∅ Sat ∅)‘∅))
4	3	eleq2d 2814	. . . . . 6 ⊢ (𝑥 = ∅ → (⟨𝐹, ∅⟩ ∈ ((∅ Sat ∅)‘𝑥) ↔ ⟨𝐹, ∅⟩ ∈ ((∅ Sat ∅)‘∅)))
5	2, 4	bibi12d 345	. . . . 5 ⊢ (𝑥 = ∅ → ((𝐹 ∈ (Fmla‘𝑥) ↔ ⟨𝐹, ∅⟩ ∈ ((∅ Sat ∅)‘𝑥)) ↔ (𝐹 ∈ (Fmla‘∅) ↔ ⟨𝐹, ∅⟩ ∈ ((∅ Sat ∅)‘∅))))
6	5	imbi2d 340	. . . 4 ⊢ (𝑥 = ∅ → ((𝐹 ∈ V → (𝐹 ∈ (Fmla‘𝑥) ↔ ⟨𝐹, ∅⟩ ∈ ((∅ Sat ∅)‘𝑥))) ↔ (𝐹 ∈ V → (𝐹 ∈ (Fmla‘∅) ↔ ⟨𝐹, ∅⟩ ∈ ((∅ Sat ∅)‘∅)))))
7		fveq2 6891	. . . . . . 7 ⊢ (𝑥 = 𝑦 → (Fmla‘𝑥) = (Fmla‘𝑦))
8	7	eleq2d 2814	. . . . . 6 ⊢ (𝑥 = 𝑦 → (𝐹 ∈ (Fmla‘𝑥) ↔ 𝐹 ∈ (Fmla‘𝑦)))
9		fveq2 6891	. . . . . . 7 ⊢ (𝑥 = 𝑦 → ((∅ Sat ∅)‘𝑥) = ((∅ Sat ∅)‘𝑦))
10	9	eleq2d 2814	. . . . . 6 ⊢ (𝑥 = 𝑦 → (⟨𝐹, ∅⟩ ∈ ((∅ Sat ∅)‘𝑥) ↔ ⟨𝐹, ∅⟩ ∈ ((∅ Sat ∅)‘𝑦)))
11	8, 10	bibi12d 345	. . . . 5 ⊢ (𝑥 = 𝑦 → ((𝐹 ∈ (Fmla‘𝑥) ↔ ⟨𝐹, ∅⟩ ∈ ((∅ Sat ∅)‘𝑥)) ↔ (𝐹 ∈ (Fmla‘𝑦) ↔ ⟨𝐹, ∅⟩ ∈ ((∅ Sat ∅)‘𝑦))))
12	11	imbi2d 340	. . . 4 ⊢ (𝑥 = 𝑦 → ((𝐹 ∈ V → (𝐹 ∈ (Fmla‘𝑥) ↔ ⟨𝐹, ∅⟩ ∈ ((∅ Sat ∅)‘𝑥))) ↔ (𝐹 ∈ V → (𝐹 ∈ (Fmla‘𝑦) ↔ ⟨𝐹, ∅⟩ ∈ ((∅ Sat ∅)‘𝑦)))))
13		fveq2 6891	. . . . . . 7 ⊢ (𝑥 = suc 𝑦 → (Fmla‘𝑥) = (Fmla‘suc 𝑦))
14	13	eleq2d 2814	. . . . . 6 ⊢ (𝑥 = suc 𝑦 → (𝐹 ∈ (Fmla‘𝑥) ↔ 𝐹 ∈ (Fmla‘suc 𝑦)))
15		fveq2 6891	. . . . . . 7 ⊢ (𝑥 = suc 𝑦 → ((∅ Sat ∅)‘𝑥) = ((∅ Sat ∅)‘suc 𝑦))
16	15	eleq2d 2814	. . . . . 6 ⊢ (𝑥 = suc 𝑦 → (⟨𝐹, ∅⟩ ∈ ((∅ Sat ∅)‘𝑥) ↔ ⟨𝐹, ∅⟩ ∈ ((∅ Sat ∅)‘suc 𝑦)))
17	14, 16	bibi12d 345	. . . . 5 ⊢ (𝑥 = suc 𝑦 → ((𝐹 ∈ (Fmla‘𝑥) ↔ ⟨𝐹, ∅⟩ ∈ ((∅ Sat ∅)‘𝑥)) ↔ (𝐹 ∈ (Fmla‘suc 𝑦) ↔ ⟨𝐹, ∅⟩ ∈ ((∅ Sat ∅)‘suc 𝑦))))
18	17	imbi2d 340	. . . 4 ⊢ (𝑥 = suc 𝑦 → ((𝐹 ∈ V → (𝐹 ∈ (Fmla‘𝑥) ↔ ⟨𝐹, ∅⟩ ∈ ((∅ Sat ∅)‘𝑥))) ↔ (𝐹 ∈ V → (𝐹 ∈ (Fmla‘suc 𝑦) ↔ ⟨𝐹, ∅⟩ ∈ ((∅ Sat ∅)‘suc 𝑦)))))
19		fveq2 6891	. . . . . . 7 ⊢ (𝑥 = 𝑁 → (Fmla‘𝑥) = (Fmla‘𝑁))
20	19	eleq2d 2814	. . . . . 6 ⊢ (𝑥 = 𝑁 → (𝐹 ∈ (Fmla‘𝑥) ↔ 𝐹 ∈ (Fmla‘𝑁)))
21		fveq2 6891	. . . . . . 7 ⊢ (𝑥 = 𝑁 → ((∅ Sat ∅)‘𝑥) = ((∅ Sat ∅)‘𝑁))
22	21	eleq2d 2814	. . . . . 6 ⊢ (𝑥 = 𝑁 → (⟨𝐹, ∅⟩ ∈ ((∅ Sat ∅)‘𝑥) ↔ ⟨𝐹, ∅⟩ ∈ ((∅ Sat ∅)‘𝑁)))
23	20, 22	bibi12d 345	. . . . 5 ⊢ (𝑥 = 𝑁 → ((𝐹 ∈ (Fmla‘𝑥) ↔ ⟨𝐹, ∅⟩ ∈ ((∅ Sat ∅)‘𝑥)) ↔ (𝐹 ∈ (Fmla‘𝑁) ↔ ⟨𝐹, ∅⟩ ∈ ((∅ Sat ∅)‘𝑁))))
24	23	imbi2d 340	. . . 4 ⊢ (𝑥 = 𝑁 → ((𝐹 ∈ V → (𝐹 ∈ (Fmla‘𝑥) ↔ ⟨𝐹, ∅⟩ ∈ ((∅ Sat ∅)‘𝑥))) ↔ (𝐹 ∈ V → (𝐹 ∈ (Fmla‘𝑁) ↔ ⟨𝐹, ∅⟩ ∈ ((∅ Sat ∅)‘𝑁)))))
25		eqeq1 2731	. . . . . . . 8 ⊢ (𝑥 = 𝐹 → (𝑥 = (𝑖∈_𝑔𝑗) ↔ 𝐹 = (𝑖∈_𝑔𝑗)))
26	25	2rexbidv 3214	. . . . . . 7 ⊢ (𝑥 = 𝐹 → (∃𝑖 ∈ ω ∃𝑗 ∈ ω 𝑥 = (𝑖∈_𝑔𝑗) ↔ ∃𝑖 ∈ ω ∃𝑗 ∈ ω 𝐹 = (𝑖∈_𝑔𝑗)))
27	26	elrab 3680	. . . . . 6 ⊢ (𝐹 ∈ {𝑥 ∈ V ∣ ∃𝑖 ∈ ω ∃𝑗 ∈ ω 𝑥 = (𝑖∈_𝑔𝑗)} ↔ (𝐹 ∈ V ∧ ∃𝑖 ∈ ω ∃𝑗 ∈ ω 𝐹 = (𝑖∈_𝑔𝑗)))
28		eqidd 2728	. . . . . . . 8 ⊢ ((𝐹 ∈ V ∧ ∃𝑖 ∈ ω ∃𝑗 ∈ ω 𝐹 = (𝑖∈_𝑔𝑗)) → ∅ = ∅)
29		simpr 484	. . . . . . . 8 ⊢ ((𝐹 ∈ V ∧ ∃𝑖 ∈ ω ∃𝑗 ∈ ω 𝐹 = (𝑖∈_𝑔𝑗)) → ∃𝑖 ∈ ω ∃𝑗 ∈ ω 𝐹 = (𝑖∈_𝑔𝑗))
30	28, 29	jca 511	. . . . . . 7 ⊢ ((𝐹 ∈ V ∧ ∃𝑖 ∈ ω ∃𝑗 ∈ ω 𝐹 = (𝑖∈_𝑔𝑗)) → (∅ = ∅ ∧ ∃𝑖 ∈ ω ∃𝑗 ∈ ω 𝐹 = (𝑖∈_𝑔𝑗)))
31		simpr 484	. . . . . . . . 9 ⊢ ((∅ = ∅ ∧ ∃𝑖 ∈ ω ∃𝑗 ∈ ω 𝐹 = (𝑖∈_𝑔𝑗)) → ∃𝑖 ∈ ω ∃𝑗 ∈ ω 𝐹 = (𝑖∈_𝑔𝑗))
32	31	anim2i 616	. . . . . . . 8 ⊢ ((𝐹 ∈ V ∧ (∅ = ∅ ∧ ∃𝑖 ∈ ω ∃𝑗 ∈ ω 𝐹 = (𝑖∈_𝑔𝑗))) → (𝐹 ∈ V ∧ ∃𝑖 ∈ ω ∃𝑗 ∈ ω 𝐹 = (𝑖∈_𝑔𝑗)))
33	32	ex 412	. . . . . . 7 ⊢ (𝐹 ∈ V → ((∅ = ∅ ∧ ∃𝑖 ∈ ω ∃𝑗 ∈ ω 𝐹 = (𝑖∈_𝑔𝑗)) → (𝐹 ∈ V ∧ ∃𝑖 ∈ ω ∃𝑗 ∈ ω 𝐹 = (𝑖∈_𝑔𝑗))))
34	30, 33	impbid2 225	. . . . . 6 ⊢ (𝐹 ∈ V → ((𝐹 ∈ V ∧ ∃𝑖 ∈ ω ∃𝑗 ∈ ω 𝐹 = (𝑖∈_𝑔𝑗)) ↔ (∅ = ∅ ∧ ∃𝑖 ∈ ω ∃𝑗 ∈ ω 𝐹 = (𝑖∈_𝑔𝑗))))
35	27, 34	bitrid 283	. . . . 5 ⊢ (𝐹 ∈ V → (𝐹 ∈ {𝑥 ∈ V ∣ ∃𝑖 ∈ ω ∃𝑗 ∈ ω 𝑥 = (𝑖∈_𝑔𝑗)} ↔ (∅ = ∅ ∧ ∃𝑖 ∈ ω ∃𝑗 ∈ ω 𝐹 = (𝑖∈_𝑔𝑗))))
36		fmla0 34928	. . . . . . 7 ⊢ (Fmla‘∅) = {𝑥 ∈ V ∣ ∃𝑖 ∈ ω ∃𝑗 ∈ ω 𝑥 = (𝑖∈_𝑔𝑗)}
37	36	eleq2i 2820	. . . . . 6 ⊢ (𝐹 ∈ (Fmla‘∅) ↔ 𝐹 ∈ {𝑥 ∈ V ∣ ∃𝑖 ∈ ω ∃𝑗 ∈ ω 𝑥 = (𝑖∈_𝑔𝑗)})
38	37	a1i 11	. . . . 5 ⊢ (𝐹 ∈ V → (𝐹 ∈ (Fmla‘∅) ↔ 𝐹 ∈ {𝑥 ∈ V ∣ ∃𝑖 ∈ ω ∃𝑗 ∈ ω 𝑥 = (𝑖∈_𝑔𝑗)}))
39		satf00 34920	. . . . . . . 8 ⊢ ((∅ Sat ∅)‘∅) = {⟨𝑥, 𝑦⟩ ∣ (𝑦 = ∅ ∧ ∃𝑖 ∈ ω ∃𝑗 ∈ ω 𝑥 = (𝑖∈_𝑔𝑗))}
40	39	a1i 11	. . . . . . 7 ⊢ (𝐹 ∈ V → ((∅ Sat ∅)‘∅) = {⟨𝑥, 𝑦⟩ ∣ (𝑦 = ∅ ∧ ∃𝑖 ∈ ω ∃𝑗 ∈ ω 𝑥 = (𝑖∈_𝑔𝑗))})
41	40	eleq2d 2814	. . . . . 6 ⊢ (𝐹 ∈ V → (⟨𝐹, ∅⟩ ∈ ((∅ Sat ∅)‘∅) ↔ ⟨𝐹, ∅⟩ ∈ {⟨𝑥, 𝑦⟩ ∣ (𝑦 = ∅ ∧ ∃𝑖 ∈ ω ∃𝑗 ∈ ω 𝑥 = (𝑖∈_𝑔𝑗))}))
42		0ex 5301	. . . . . . 7 ⊢ ∅ ∈ V
43		eqeq1 2731	. . . . . . . . 9 ⊢ (𝑦 = ∅ → (𝑦 = ∅ ↔ ∅ = ∅))
44	43, 26	bi2anan9r 638	. . . . . . . 8 ⊢ ((𝑥 = 𝐹 ∧ 𝑦 = ∅) → ((𝑦 = ∅ ∧ ∃𝑖 ∈ ω ∃𝑗 ∈ ω 𝑥 = (𝑖∈_𝑔𝑗)) ↔ (∅ = ∅ ∧ ∃𝑖 ∈ ω ∃𝑗 ∈ ω 𝐹 = (𝑖∈_𝑔𝑗))))
45	44	opelopabga 5529	. . . . . . 7 ⊢ ((𝐹 ∈ V ∧ ∅ ∈ V) → (⟨𝐹, ∅⟩ ∈ {⟨𝑥, 𝑦⟩ ∣ (𝑦 = ∅ ∧ ∃𝑖 ∈ ω ∃𝑗 ∈ ω 𝑥 = (𝑖∈_𝑔𝑗))} ↔ (∅ = ∅ ∧ ∃𝑖 ∈ ω ∃𝑗 ∈ ω 𝐹 = (𝑖∈_𝑔𝑗))))
46	42, 45	mpan2 690	. . . . . 6 ⊢ (𝐹 ∈ V → (⟨𝐹, ∅⟩ ∈ {⟨𝑥, 𝑦⟩ ∣ (𝑦 = ∅ ∧ ∃𝑖 ∈ ω ∃𝑗 ∈ ω 𝑥 = (𝑖∈_𝑔𝑗))} ↔ (∅ = ∅ ∧ ∃𝑖 ∈ ω ∃𝑗 ∈ ω 𝐹 = (𝑖∈_𝑔𝑗))))
47	41, 46	bitrd 279	. . . . 5 ⊢ (𝐹 ∈ V → (⟨𝐹, ∅⟩ ∈ ((∅ Sat ∅)‘∅) ↔ (∅ = ∅ ∧ ∃𝑖 ∈ ω ∃𝑗 ∈ ω 𝐹 = (𝑖∈_𝑔𝑗))))
48	35, 38, 47	3bitr4d 311	. . . 4 ⊢ (𝐹 ∈ V → (𝐹 ∈ (Fmla‘∅) ↔ ⟨𝐹, ∅⟩ ∈ ((∅ Sat ∅)‘∅)))
49		eqid 2727	. . . . . . . . . . . 12 ⊢ ∅ = ∅
50	49	biantrur 530	. . . . . . . . . . 11 ⊢ (∃𝑢 ∈ ((∅ Sat ∅)‘𝑦)(∃𝑣 ∈ ((∅ Sat ∅)‘𝑦)𝐹 = ((1^st ‘𝑢)⊼_𝑔(1^st ‘𝑣)) ∨ ∃𝑖 ∈ ω 𝐹 = ∀_𝑔𝑖(1^st ‘𝑢)) ↔ (∅ = ∅ ∧ ∃𝑢 ∈ ((∅ Sat ∅)‘𝑦)(∃𝑣 ∈ ((∅ Sat ∅)‘𝑦)𝐹 = ((1^st ‘𝑢)⊼_𝑔(1^st ‘𝑣)) ∨ ∃𝑖 ∈ ω 𝐹 = ∀_𝑔𝑖(1^st ‘𝑢))))
51	50	bicomi 223	. . . . . . . . . 10 ⊢ ((∅ = ∅ ∧ ∃𝑢 ∈ ((∅ Sat ∅)‘𝑦)(∃𝑣 ∈ ((∅ Sat ∅)‘𝑦)𝐹 = ((1^st ‘𝑢)⊼_𝑔(1^st ‘𝑣)) ∨ ∃𝑖 ∈ ω 𝐹 = ∀_𝑔𝑖(1^st ‘𝑢))) ↔ ∃𝑢 ∈ ((∅ Sat ∅)‘𝑦)(∃𝑣 ∈ ((∅ Sat ∅)‘𝑦)𝐹 = ((1^st ‘𝑢)⊼_𝑔(1^st ‘𝑣)) ∨ ∃𝑖 ∈ ω 𝐹 = ∀_𝑔𝑖(1^st ‘𝑢)))
52	51	a1i 11	. . . . . . . . 9 ⊢ (𝐹 ∈ V → ((∅ = ∅ ∧ ∃𝑢 ∈ ((∅ Sat ∅)‘𝑦)(∃𝑣 ∈ ((∅ Sat ∅)‘𝑦)𝐹 = ((1^st ‘𝑢)⊼_𝑔(1^st ‘𝑣)) ∨ ∃𝑖 ∈ ω 𝐹 = ∀_𝑔𝑖(1^st ‘𝑢))) ↔ ∃𝑢 ∈ ((∅ Sat ∅)‘𝑦)(∃𝑣 ∈ ((∅ Sat ∅)‘𝑦)𝐹 = ((1^st ‘𝑢)⊼_𝑔(1^st ‘𝑣)) ∨ ∃𝑖 ∈ ω 𝐹 = ∀_𝑔𝑖(1^st ‘𝑢))))
53		eqeq1 2731	. . . . . . . . . . . 12 ⊢ (𝑧 = ∅ → (𝑧 = ∅ ↔ ∅ = ∅))
54		eqeq1 2731	. . . . . . . . . . . . . . 15 ⊢ (𝑥 = 𝐹 → (𝑥 = ((1^st ‘𝑢)⊼_𝑔(1^st ‘𝑣)) ↔ 𝐹 = ((1^st ‘𝑢)⊼_𝑔(1^st ‘𝑣))))
55	54	rexbidv 3173	. . . . . . . . . . . . . 14 ⊢ (𝑥 = 𝐹 → (∃𝑣 ∈ ((∅ Sat ∅)‘𝑦)𝑥 = ((1^st ‘𝑢)⊼_𝑔(1^st ‘𝑣)) ↔ ∃𝑣 ∈ ((∅ Sat ∅)‘𝑦)𝐹 = ((1^st ‘𝑢)⊼_𝑔(1^st ‘𝑣))))
56		eqeq1 2731	. . . . . . . . . . . . . . 15 ⊢ (𝑥 = 𝐹 → (𝑥 = ∀_𝑔𝑖(1^st ‘𝑢) ↔ 𝐹 = ∀_𝑔𝑖(1^st ‘𝑢)))
57	56	rexbidv 3173	. . . . . . . . . . . . . 14 ⊢ (𝑥 = 𝐹 → (∃𝑖 ∈ ω 𝑥 = ∀_𝑔𝑖(1^st ‘𝑢) ↔ ∃𝑖 ∈ ω 𝐹 = ∀_𝑔𝑖(1^st ‘𝑢)))
58	55, 57	orbi12d 917	. . . . . . . . . . . . 13 ⊢ (𝑥 = 𝐹 → ((∃𝑣 ∈ ((∅ Sat ∅)‘𝑦)𝑥 = ((1^st ‘𝑢)⊼_𝑔(1^st ‘𝑣)) ∨ ∃𝑖 ∈ ω 𝑥 = ∀_𝑔𝑖(1^st ‘𝑢)) ↔ (∃𝑣 ∈ ((∅ Sat ∅)‘𝑦)𝐹 = ((1^st ‘𝑢)⊼_𝑔(1^st ‘𝑣)) ∨ ∃𝑖 ∈ ω 𝐹 = ∀_𝑔𝑖(1^st ‘𝑢))))
59	58	rexbidv 3173	. . . . . . . . . . . 12 ⊢ (𝑥 = 𝐹 → (∃𝑢 ∈ ((∅ Sat ∅)‘𝑦)(∃𝑣 ∈ ((∅ Sat ∅)‘𝑦)𝑥 = ((1^st ‘𝑢)⊼_𝑔(1^st ‘𝑣)) ∨ ∃𝑖 ∈ ω 𝑥 = ∀_𝑔𝑖(1^st ‘𝑢)) ↔ ∃𝑢 ∈ ((∅ Sat ∅)‘𝑦)(∃𝑣 ∈ ((∅ Sat ∅)‘𝑦)𝐹 = ((1^st ‘𝑢)⊼_𝑔(1^st ‘𝑣)) ∨ ∃𝑖 ∈ ω 𝐹 = ∀_𝑔𝑖(1^st ‘𝑢))))
60	53, 59	bi2anan9r 638	. . . . . . . . . . 11 ⊢ ((𝑥 = 𝐹 ∧ 𝑧 = ∅) → ((𝑧 = ∅ ∧ ∃𝑢 ∈ ((∅ Sat ∅)‘𝑦)(∃𝑣 ∈ ((∅ Sat ∅)‘𝑦)𝑥 = ((1^st ‘𝑢)⊼_𝑔(1^st ‘𝑣)) ∨ ∃𝑖 ∈ ω 𝑥 = ∀_𝑔𝑖(1^st ‘𝑢))) ↔ (∅ = ∅ ∧ ∃𝑢 ∈ ((∅ Sat ∅)‘𝑦)(∃𝑣 ∈ ((∅ Sat ∅)‘𝑦)𝐹 = ((1^st ‘𝑢)⊼_𝑔(1^st ‘𝑣)) ∨ ∃𝑖 ∈ ω 𝐹 = ∀_𝑔𝑖(1^st ‘𝑢)))))
61	60	opelopabga 5529	. . . . . . . . . 10 ⊢ ((𝐹 ∈ V ∧ ∅ ∈ V) → (⟨𝐹, ∅⟩ ∈ {⟨𝑥, 𝑧⟩ ∣ (𝑧 = ∅ ∧ ∃𝑢 ∈ ((∅ Sat ∅)‘𝑦)(∃𝑣 ∈ ((∅ Sat ∅)‘𝑦)𝑥 = ((1^st ‘𝑢)⊼_𝑔(1^st ‘𝑣)) ∨ ∃𝑖 ∈ ω 𝑥 = ∀_𝑔𝑖(1^st ‘𝑢)))} ↔ (∅ = ∅ ∧ ∃𝑢 ∈ ((∅ Sat ∅)‘𝑦)(∃𝑣 ∈ ((∅ Sat ∅)‘𝑦)𝐹 = ((1^st ‘𝑢)⊼_𝑔(1^st ‘𝑣)) ∨ ∃𝑖 ∈ ω 𝐹 = ∀_𝑔𝑖(1^st ‘𝑢)))))
62	42, 61	mpan2 690	. . . . . . . . 9 ⊢ (𝐹 ∈ V → (⟨𝐹, ∅⟩ ∈ {⟨𝑥, 𝑧⟩ ∣ (𝑧 = ∅ ∧ ∃𝑢 ∈ ((∅ Sat ∅)‘𝑦)(∃𝑣 ∈ ((∅ Sat ∅)‘𝑦)𝑥 = ((1^st ‘𝑢)⊼_𝑔(1^st ‘𝑣)) ∨ ∃𝑖 ∈ ω 𝑥 = ∀_𝑔𝑖(1^st ‘𝑢)))} ↔ (∅ = ∅ ∧ ∃𝑢 ∈ ((∅ Sat ∅)‘𝑦)(∃𝑣 ∈ ((∅ Sat ∅)‘𝑦)𝐹 = ((1^st ‘𝑢)⊼_𝑔(1^st ‘𝑣)) ∨ ∃𝑖 ∈ ω 𝐹 = ∀_𝑔𝑖(1^st ‘𝑢)))))
63	59	elabg 3663	. . . . . . . . 9 ⊢ (𝐹 ∈ V → (𝐹 ∈ {𝑥 ∣ ∃𝑢 ∈ ((∅ Sat ∅)‘𝑦)(∃𝑣 ∈ ((∅ Sat ∅)‘𝑦)𝑥 = ((1^st ‘𝑢)⊼_𝑔(1^st ‘𝑣)) ∨ ∃𝑖 ∈ ω 𝑥 = ∀_𝑔𝑖(1^st ‘𝑢))} ↔ ∃𝑢 ∈ ((∅ Sat ∅)‘𝑦)(∃𝑣 ∈ ((∅ Sat ∅)‘𝑦)𝐹 = ((1^st ‘𝑢)⊼_𝑔(1^st ‘𝑣)) ∨ ∃𝑖 ∈ ω 𝐹 = ∀_𝑔𝑖(1^st ‘𝑢))))
64	52, 62, 63	3bitr4d 311	. . . . . . . 8 ⊢ (𝐹 ∈ V → (⟨𝐹, ∅⟩ ∈ {⟨𝑥, 𝑧⟩ ∣ (𝑧 = ∅ ∧ ∃𝑢 ∈ ((∅ Sat ∅)‘𝑦)(∃𝑣 ∈ ((∅ Sat ∅)‘𝑦)𝑥 = ((1^st ‘𝑢)⊼_𝑔(1^st ‘𝑣)) ∨ ∃𝑖 ∈ ω 𝑥 = ∀_𝑔𝑖(1^st ‘𝑢)))} ↔ 𝐹 ∈ {𝑥 ∣ ∃𝑢 ∈ ((∅ Sat ∅)‘𝑦)(∃𝑣 ∈ ((∅ Sat ∅)‘𝑦)𝑥 = ((1^st ‘𝑢)⊼_𝑔(1^st ‘𝑣)) ∨ ∃𝑖 ∈ ω 𝑥 = ∀_𝑔𝑖(1^st ‘𝑢))}))
65	64	adantl 481	. . . . . . 7 ⊢ (((𝑦 ∈ ω ∧ (𝐹 ∈ V → (𝐹 ∈ (Fmla‘𝑦) ↔ ⟨𝐹, ∅⟩ ∈ ((∅ Sat ∅)‘𝑦)))) ∧ 𝐹 ∈ V) → (⟨𝐹, ∅⟩ ∈ {⟨𝑥, 𝑧⟩ ∣ (𝑧 = ∅ ∧ ∃𝑢 ∈ ((∅ Sat ∅)‘𝑦)(∃𝑣 ∈ ((∅ Sat ∅)‘𝑦)𝑥 = ((1^st ‘𝑢)⊼_𝑔(1^st ‘𝑣)) ∨ ∃𝑖 ∈ ω 𝑥 = ∀_𝑔𝑖(1^st ‘𝑢)))} ↔ 𝐹 ∈ {𝑥 ∣ ∃𝑢 ∈ ((∅ Sat ∅)‘𝑦)(∃𝑣 ∈ ((∅ Sat ∅)‘𝑦)𝑥 = ((1^st ‘𝑢)⊼_𝑔(1^st ‘𝑣)) ∨ ∃𝑖 ∈ ω 𝑥 = ∀_𝑔𝑖(1^st ‘𝑢))}))
66	65	orbi2d 914	. . . . . 6 ⊢ (((𝑦 ∈ ω ∧ (𝐹 ∈ V → (𝐹 ∈ (Fmla‘𝑦) ↔ ⟨𝐹, ∅⟩ ∈ ((∅ Sat ∅)‘𝑦)))) ∧ 𝐹 ∈ V) → ((⟨𝐹, ∅⟩ ∈ ((∅ Sat ∅)‘𝑦) ∨ ⟨𝐹, ∅⟩ ∈ {⟨𝑥, 𝑧⟩ ∣ (𝑧 = ∅ ∧ ∃𝑢 ∈ ((∅ Sat ∅)‘𝑦)(∃𝑣 ∈ ((∅ Sat ∅)‘𝑦)𝑥 = ((1^st ‘𝑢)⊼_𝑔(1^st ‘𝑣)) ∨ ∃𝑖 ∈ ω 𝑥 = ∀_𝑔𝑖(1^st ‘𝑢)))}) ↔ (⟨𝐹, ∅⟩ ∈ ((∅ Sat ∅)‘𝑦) ∨ 𝐹 ∈ {𝑥 ∣ ∃𝑢 ∈ ((∅ Sat ∅)‘𝑦)(∃𝑣 ∈ ((∅ Sat ∅)‘𝑦)𝑥 = ((1^st ‘𝑢)⊼_𝑔(1^st ‘𝑣)) ∨ ∃𝑖 ∈ ω 𝑥 = ∀_𝑔𝑖(1^st ‘𝑢))})))
67		eqid 2727	. . . . . . . . . 10 ⊢ (∅ Sat ∅) = (∅ Sat ∅)
68	67	satf0suc 34922	. . . . . . . . 9 ⊢ (𝑦 ∈ ω → ((∅ Sat ∅)‘suc 𝑦) = (((∅ Sat ∅)‘𝑦) ∪ {⟨𝑥, 𝑧⟩ ∣ (𝑧 = ∅ ∧ ∃𝑢 ∈ ((∅ Sat ∅)‘𝑦)(∃𝑣 ∈ ((∅ Sat ∅)‘𝑦)𝑥 = ((1^st ‘𝑢)⊼_𝑔(1^st ‘𝑣)) ∨ ∃𝑖 ∈ ω 𝑥 = ∀_𝑔𝑖(1^st ‘𝑢)))}))
69	68	eleq2d 2814	. . . . . . . 8 ⊢ (𝑦 ∈ ω → (⟨𝐹, ∅⟩ ∈ ((∅ Sat ∅)‘suc 𝑦) ↔ ⟨𝐹, ∅⟩ ∈ (((∅ Sat ∅)‘𝑦) ∪ {⟨𝑥, 𝑧⟩ ∣ (𝑧 = ∅ ∧ ∃𝑢 ∈ ((∅ Sat ∅)‘𝑦)(∃𝑣 ∈ ((∅ Sat ∅)‘𝑦)𝑥 = ((1^st ‘𝑢)⊼_𝑔(1^st ‘𝑣)) ∨ ∃𝑖 ∈ ω 𝑥 = ∀_𝑔𝑖(1^st ‘𝑢)))})))
70		elun 4144	. . . . . . . 8 ⊢ (⟨𝐹, ∅⟩ ∈ (((∅ Sat ∅)‘𝑦) ∪ {⟨𝑥, 𝑧⟩ ∣ (𝑧 = ∅ ∧ ∃𝑢 ∈ ((∅ Sat ∅)‘𝑦)(∃𝑣 ∈ ((∅ Sat ∅)‘𝑦)𝑥 = ((1^st ‘𝑢)⊼_𝑔(1^st ‘𝑣)) ∨ ∃𝑖 ∈ ω 𝑥 = ∀_𝑔𝑖(1^st ‘𝑢)))}) ↔ (⟨𝐹, ∅⟩ ∈ ((∅ Sat ∅)‘𝑦) ∨ ⟨𝐹, ∅⟩ ∈ {⟨𝑥, 𝑧⟩ ∣ (𝑧 = ∅ ∧ ∃𝑢 ∈ ((∅ Sat ∅)‘𝑦)(∃𝑣 ∈ ((∅ Sat ∅)‘𝑦)𝑥 = ((1^st ‘𝑢)⊼_𝑔(1^st ‘𝑣)) ∨ ∃𝑖 ∈ ω 𝑥 = ∀_𝑔𝑖(1^st ‘𝑢)))}))
71	69, 70	bitrdi 287	. . . . . . 7 ⊢ (𝑦 ∈ ω → (⟨𝐹, ∅⟩ ∈ ((∅ Sat ∅)‘suc 𝑦) ↔ (⟨𝐹, ∅⟩ ∈ ((∅ Sat ∅)‘𝑦) ∨ ⟨𝐹, ∅⟩ ∈ {⟨𝑥, 𝑧⟩ ∣ (𝑧 = ∅ ∧ ∃𝑢 ∈ ((∅ Sat ∅)‘𝑦)(∃𝑣 ∈ ((∅ Sat ∅)‘𝑦)𝑥 = ((1^st ‘𝑢)⊼_𝑔(1^st ‘𝑣)) ∨ ∃𝑖 ∈ ω 𝑥 = ∀_𝑔𝑖(1^st ‘𝑢)))})))
72	71	ad2antrr 725	. . . . . 6 ⊢ (((𝑦 ∈ ω ∧ (𝐹 ∈ V → (𝐹 ∈ (Fmla‘𝑦) ↔ ⟨𝐹, ∅⟩ ∈ ((∅ Sat ∅)‘𝑦)))) ∧ 𝐹 ∈ V) → (⟨𝐹, ∅⟩ ∈ ((∅ Sat ∅)‘suc 𝑦) ↔ (⟨𝐹, ∅⟩ ∈ ((∅ Sat ∅)‘𝑦) ∨ ⟨𝐹, ∅⟩ ∈ {⟨𝑥, 𝑧⟩ ∣ (𝑧 = ∅ ∧ ∃𝑢 ∈ ((∅ Sat ∅)‘𝑦)(∃𝑣 ∈ ((∅ Sat ∅)‘𝑦)𝑥 = ((1^st ‘𝑢)⊼_𝑔(1^st ‘𝑣)) ∨ ∃𝑖 ∈ ω 𝑥 = ∀_𝑔𝑖(1^st ‘𝑢)))})))
73		fmlasuc0 34930	. . . . . . . . 9 ⊢ (𝑦 ∈ ω → (Fmla‘suc 𝑦) = ((Fmla‘𝑦) ∪ {𝑥 ∣ ∃𝑢 ∈ ((∅ Sat ∅)‘𝑦)(∃𝑣 ∈ ((∅ Sat ∅)‘𝑦)𝑥 = ((1^st ‘𝑢)⊼_𝑔(1^st ‘𝑣)) ∨ ∃𝑖 ∈ ω 𝑥 = ∀_𝑔𝑖(1^st ‘𝑢))}))
74	73	eleq2d 2814	. . . . . . . 8 ⊢ (𝑦 ∈ ω → (𝐹 ∈ (Fmla‘suc 𝑦) ↔ 𝐹 ∈ ((Fmla‘𝑦) ∪ {𝑥 ∣ ∃𝑢 ∈ ((∅ Sat ∅)‘𝑦)(∃𝑣 ∈ ((∅ Sat ∅)‘𝑦)𝑥 = ((1^st ‘𝑢)⊼_𝑔(1^st ‘𝑣)) ∨ ∃𝑖 ∈ ω 𝑥 = ∀_𝑔𝑖(1^st ‘𝑢))})))
75	74	ad2antrr 725	. . . . . . 7 ⊢ (((𝑦 ∈ ω ∧ (𝐹 ∈ V → (𝐹 ∈ (Fmla‘𝑦) ↔ ⟨𝐹, ∅⟩ ∈ ((∅ Sat ∅)‘𝑦)))) ∧ 𝐹 ∈ V) → (𝐹 ∈ (Fmla‘suc 𝑦) ↔ 𝐹 ∈ ((Fmla‘𝑦) ∪ {𝑥 ∣ ∃𝑢 ∈ ((∅ Sat ∅)‘𝑦)(∃𝑣 ∈ ((∅ Sat ∅)‘𝑦)𝑥 = ((1^st ‘𝑢)⊼_𝑔(1^st ‘𝑣)) ∨ ∃𝑖 ∈ ω 𝑥 = ∀_𝑔𝑖(1^st ‘𝑢))})))
76		elun 4144	. . . . . . . 8 ⊢ (𝐹 ∈ ((Fmla‘𝑦) ∪ {𝑥 ∣ ∃𝑢 ∈ ((∅ Sat ∅)‘𝑦)(∃𝑣 ∈ ((∅ Sat ∅)‘𝑦)𝑥 = ((1^st ‘𝑢)⊼_𝑔(1^st ‘𝑣)) ∨ ∃𝑖 ∈ ω 𝑥 = ∀_𝑔𝑖(1^st ‘𝑢))}) ↔ (𝐹 ∈ (Fmla‘𝑦) ∨ 𝐹 ∈ {𝑥 ∣ ∃𝑢 ∈ ((∅ Sat ∅)‘𝑦)(∃𝑣 ∈ ((∅ Sat ∅)‘𝑦)𝑥 = ((1^st ‘𝑢)⊼_𝑔(1^st ‘𝑣)) ∨ ∃𝑖 ∈ ω 𝑥 = ∀_𝑔𝑖(1^st ‘𝑢))}))
77	76	a1i 11	. . . . . . 7 ⊢ (((𝑦 ∈ ω ∧ (𝐹 ∈ V → (𝐹 ∈ (Fmla‘𝑦) ↔ ⟨𝐹, ∅⟩ ∈ ((∅ Sat ∅)‘𝑦)))) ∧ 𝐹 ∈ V) → (𝐹 ∈ ((Fmla‘𝑦) ∪ {𝑥 ∣ ∃𝑢 ∈ ((∅ Sat ∅)‘𝑦)(∃𝑣 ∈ ((∅ Sat ∅)‘𝑦)𝑥 = ((1^st ‘𝑢)⊼_𝑔(1^st ‘𝑣)) ∨ ∃𝑖 ∈ ω 𝑥 = ∀_𝑔𝑖(1^st ‘𝑢))}) ↔ (𝐹 ∈ (Fmla‘𝑦) ∨ 𝐹 ∈ {𝑥 ∣ ∃𝑢 ∈ ((∅ Sat ∅)‘𝑦)(∃𝑣 ∈ ((∅ Sat ∅)‘𝑦)𝑥 = ((1^st ‘𝑢)⊼_𝑔(1^st ‘𝑣)) ∨ ∃𝑖 ∈ ω 𝑥 = ∀_𝑔𝑖(1^st ‘𝑢))})))
78		simpr 484	. . . . . . . . 9 ⊢ ((𝑦 ∈ ω ∧ (𝐹 ∈ V → (𝐹 ∈ (Fmla‘𝑦) ↔ ⟨𝐹, ∅⟩ ∈ ((∅ Sat ∅)‘𝑦)))) → (𝐹 ∈ V → (𝐹 ∈ (Fmla‘𝑦) ↔ ⟨𝐹, ∅⟩ ∈ ((∅ Sat ∅)‘𝑦))))
79	78	imp 406	. . . . . . . 8 ⊢ (((𝑦 ∈ ω ∧ (𝐹 ∈ V → (𝐹 ∈ (Fmla‘𝑦) ↔ ⟨𝐹, ∅⟩ ∈ ((∅ Sat ∅)‘𝑦)))) ∧ 𝐹 ∈ V) → (𝐹 ∈ (Fmla‘𝑦) ↔ ⟨𝐹, ∅⟩ ∈ ((∅ Sat ∅)‘𝑦)))
80	79	orbi1d 915	. . . . . . 7 ⊢ (((𝑦 ∈ ω ∧ (𝐹 ∈ V → (𝐹 ∈ (Fmla‘𝑦) ↔ ⟨𝐹, ∅⟩ ∈ ((∅ Sat ∅)‘𝑦)))) ∧ 𝐹 ∈ V) → ((𝐹 ∈ (Fmla‘𝑦) ∨ 𝐹 ∈ {𝑥 ∣ ∃𝑢 ∈ ((∅ Sat ∅)‘𝑦)(∃𝑣 ∈ ((∅ Sat ∅)‘𝑦)𝑥 = ((1^st ‘𝑢)⊼_𝑔(1^st ‘𝑣)) ∨ ∃𝑖 ∈ ω 𝑥 = ∀_𝑔𝑖(1^st ‘𝑢))}) ↔ (⟨𝐹, ∅⟩ ∈ ((∅ Sat ∅)‘𝑦) ∨ 𝐹 ∈ {𝑥 ∣ ∃𝑢 ∈ ((∅ Sat ∅)‘𝑦)(∃𝑣 ∈ ((∅ Sat ∅)‘𝑦)𝑥 = ((1^st ‘𝑢)⊼_𝑔(1^st ‘𝑣)) ∨ ∃𝑖 ∈ ω 𝑥 = ∀_𝑔𝑖(1^st ‘𝑢))})))
81	75, 77, 80	3bitrd 305	. . . . . 6 ⊢ (((𝑦 ∈ ω ∧ (𝐹 ∈ V → (𝐹 ∈ (Fmla‘𝑦) ↔ ⟨𝐹, ∅⟩ ∈ ((∅ Sat ∅)‘𝑦)))) ∧ 𝐹 ∈ V) → (𝐹 ∈ (Fmla‘suc 𝑦) ↔ (⟨𝐹, ∅⟩ ∈ ((∅ Sat ∅)‘𝑦) ∨ 𝐹 ∈ {𝑥 ∣ ∃𝑢 ∈ ((∅ Sat ∅)‘𝑦)(∃𝑣 ∈ ((∅ Sat ∅)‘𝑦)𝑥 = ((1^st ‘𝑢)⊼_𝑔(1^st ‘𝑣)) ∨ ∃𝑖 ∈ ω 𝑥 = ∀_𝑔𝑖(1^st ‘𝑢))})))
82	66, 72, 81	3bitr4rd 312	. . . . 5 ⊢ (((𝑦 ∈ ω ∧ (𝐹 ∈ V → (𝐹 ∈ (Fmla‘𝑦) ↔ ⟨𝐹, ∅⟩ ∈ ((∅ Sat ∅)‘𝑦)))) ∧ 𝐹 ∈ V) → (𝐹 ∈ (Fmla‘suc 𝑦) ↔ ⟨𝐹, ∅⟩ ∈ ((∅ Sat ∅)‘suc 𝑦)))
83	82	exp31 419	. . . 4 ⊢ (𝑦 ∈ ω → ((𝐹 ∈ V → (𝐹 ∈ (Fmla‘𝑦) ↔ ⟨𝐹, ∅⟩ ∈ ((∅ Sat ∅)‘𝑦))) → (𝐹 ∈ V → (𝐹 ∈ (Fmla‘suc 𝑦) ↔ ⟨𝐹, ∅⟩ ∈ ((∅ Sat ∅)‘suc 𝑦)))))
84	6, 12, 18, 24, 48, 83	finds 7898	. . 3 ⊢ (𝑁 ∈ ω → (𝐹 ∈ V → (𝐹 ∈ (Fmla‘𝑁) ↔ ⟨𝐹, ∅⟩ ∈ ((∅ Sat ∅)‘𝑁))))
85	84	com12 32	. 2 ⊢ (𝐹 ∈ V → (𝑁 ∈ ω → (𝐹 ∈ (Fmla‘𝑁) ↔ ⟨𝐹, ∅⟩ ∈ ((∅ Sat ∅)‘𝑁))))
86		prcnel 3493	. . . . 5 ⊢ (¬ 𝐹 ∈ V → ¬ 𝐹 ∈ (Fmla‘𝑁))
87	86	adantr 480	. . . 4 ⊢ ((¬ 𝐹 ∈ V ∧ 𝑁 ∈ ω) → ¬ 𝐹 ∈ (Fmla‘𝑁))
88		opprc1 4893	. . . . . 6 ⊢ (¬ 𝐹 ∈ V → ⟨𝐹, ∅⟩ = ∅)
89	88	adantr 480	. . . . 5 ⊢ ((¬ 𝐹 ∈ V ∧ 𝑁 ∈ ω) → ⟨𝐹, ∅⟩ = ∅)
90		satf0n0 34924	. . . . . . 7 ⊢ (𝑁 ∈ ω → ∅ ∉ ((∅ Sat ∅)‘𝑁))
91		df-nel 3042	. . . . . . 7 ⊢ (∅ ∉ ((∅ Sat ∅)‘𝑁) ↔ ¬ ∅ ∈ ((∅ Sat ∅)‘𝑁))
92	90, 91	sylib 217	. . . . . 6 ⊢ (𝑁 ∈ ω → ¬ ∅ ∈ ((∅ Sat ∅)‘𝑁))
93	92	adantl 481	. . . . 5 ⊢ ((¬ 𝐹 ∈ V ∧ 𝑁 ∈ ω) → ¬ ∅ ∈ ((∅ Sat ∅)‘𝑁))
94	89, 93	eqneltrd 2848	. . . 4 ⊢ ((¬ 𝐹 ∈ V ∧ 𝑁 ∈ ω) → ¬ ⟨𝐹, ∅⟩ ∈ ((∅ Sat ∅)‘𝑁))
95	87, 94	2falsed 376	. . 3 ⊢ ((¬ 𝐹 ∈ V ∧ 𝑁 ∈ ω) → (𝐹 ∈ (Fmla‘𝑁) ↔ ⟨𝐹, ∅⟩ ∈ ((∅ Sat ∅)‘𝑁)))
96	95	ex 412	. 2 ⊢ (¬ 𝐹 ∈ V → (𝑁 ∈ ω → (𝐹 ∈ (Fmla‘𝑁) ↔ ⟨𝐹, ∅⟩ ∈ ((∅ Sat ∅)‘𝑁))))
97	85, 96	pm2.61i 182	1 ⊢ (𝑁 ∈ ω → (𝐹 ∈ (Fmla‘𝑁) ↔ ⟨𝐹, ∅⟩ ∈ ((∅ Sat ∅)‘𝑁)))

Colors of variables: wff setvar class

Syntax hints: ¬ wn 3 → wi 4 ↔ wb 205 ∧ wa 395 ∨ wo 846 = wceq 1534 ∈ wcel 2099 {cab 2704 ∉ wnel 3041 ∃wrex 3065 {crab 3427 Vcvv 3469 ∪ cun 3942 ∅c0 4318 ⟨cop 4630 {copab 5204 suc csuc 6365 ‘cfv 6542 (class class class)co 7414 ωcom 7864 1^st c1st 7985 ∈_𝑔cgoe 34879 ⊼_𝑔cgna 34880 ∀_𝑔cgol 34881 Sat csat 34882 Fmlacfmla 34883

This theorem was proved from axioms: ax-mp 5 ax-1 6 ax-2 7 ax-3 8 ax-gen 1790 ax-4 1804 ax-5 1906 ax-6 1964 ax-7 2004 ax-8 2101 ax-9 2109 ax-10 2130 ax-11 2147 ax-12 2164 ax-ext 2698 ax-rep 5279 ax-sep 5293 ax-nul 5300 ax-pow 5359 ax-pr 5423 ax-un 7734 ax-inf2 9656

This theorem depends on definitions: df-bi 206 df-an 396 df-or 847 df-3or 1086 df-3an 1087 df-tru 1537 df-fal 1547 df-ex 1775 df-nf 1779 df-sb 2061 df-mo 2529 df-eu 2558 df-clab 2705 df-cleq 2719 df-clel 2805 df-nfc 2880 df-ne 2936 df-nel 3042 df-ral 3057 df-rex 3066 df-reu 3372 df-rab 3428 df-v 3471 df-sbc 3775 df-csb 3890 df-dif 3947 df-un 3949 df-in 3951 df-ss 3961 df-pss 3963 df-nul 4319 df-if 4525 df-pw 4600 df-sn 4625 df-pr 4627 df-op 4631 df-uni 4904 df-iun 4993 df-br 5143 df-opab 5205 df-mpt 5226 df-tr 5260 df-id 5570 df-eprel 5576 df-po 5584 df-so 5585 df-fr 5627 df-we 5629 df-xp 5678 df-rel 5679 df-cnv 5680 df-co 5681 df-dm 5682 df-rn 5683 df-res 5684 df-ima 5685 df-pred 6299 df-ord 6366 df-on 6367 df-lim 6368 df-suc 6369 df-iota 6494 df-fun 6544 df-fn 6545 df-f 6546 df-f1 6547 df-fo 6548 df-f1o 6549 df-fv 6550 df-ov 7417 df-oprab 7418 df-mpo 7419 df-om 7865 df-1st 7987 df-2nd 7988 df-frecs 8280 df-wrecs 8311 df-recs 8385 df-rdg 8424 df-map 8838 df-goel 34886 df-sat 34889 df-fmla 34891

This theorem is referenced by: fmlasuc 34932

W3C validator