| Step | Hyp | Ref
 | Expression | 
| 1 |   | rexeq 2694 | 
. . 3
⊢ (𝑤 = ∅ → (∃𝑥 ∈ 𝑤 𝜑 ↔ ∃𝑥 ∈ ∅ 𝜑)) | 
| 2 | 1 | dcbid 839 | 
. 2
⊢ (𝑤 = ∅ →
(DECID ∃𝑥 ∈ 𝑤 𝜑 ↔ DECID ∃𝑥 ∈ ∅ 𝜑)) | 
| 3 |   | rexeq 2694 | 
. . 3
⊢ (𝑤 = 𝑦 → (∃𝑥 ∈ 𝑤 𝜑 ↔ ∃𝑥 ∈ 𝑦 𝜑)) | 
| 4 | 3 | dcbid 839 | 
. 2
⊢ (𝑤 = 𝑦 → (DECID ∃𝑥 ∈ 𝑤 𝜑 ↔ DECID ∃𝑥 ∈ 𝑦 𝜑)) | 
| 5 |   | rexeq 2694 | 
. . 3
⊢ (𝑤 = (𝑦 ∪ {𝑧}) → (∃𝑥 ∈ 𝑤 𝜑 ↔ ∃𝑥 ∈ (𝑦 ∪ {𝑧})𝜑)) | 
| 6 | 5 | dcbid 839 | 
. 2
⊢ (𝑤 = (𝑦 ∪ {𝑧}) → (DECID ∃𝑥 ∈ 𝑤 𝜑 ↔ DECID ∃𝑥 ∈ (𝑦 ∪ {𝑧})𝜑)) | 
| 7 |   | rexeq 2694 | 
. . 3
⊢ (𝑤 = 𝐴 → (∃𝑥 ∈ 𝑤 𝜑 ↔ ∃𝑥 ∈ 𝐴 𝜑)) | 
| 8 | 7 | dcbid 839 | 
. 2
⊢ (𝑤 = 𝐴 → (DECID ∃𝑥 ∈ 𝑤 𝜑 ↔ DECID ∃𝑥 ∈ 𝐴 𝜑)) | 
| 9 |   | rex0 3468 | 
. . . . 5
⊢  ¬
∃𝑥 ∈ ∅
𝜑 | 
| 10 | 9 | olci 733 | 
. . . 4
⊢
(∃𝑥 ∈
∅ 𝜑 ∨ ¬
∃𝑥 ∈ ∅
𝜑) | 
| 11 |   | df-dc 836 | 
. . . 4
⊢
(DECID ∃𝑥 ∈ ∅ 𝜑 ↔ (∃𝑥 ∈ ∅ 𝜑 ∨ ¬ ∃𝑥 ∈ ∅ 𝜑)) | 
| 12 | 10, 11 | mpbir 146 | 
. . 3
⊢
DECID ∃𝑥 ∈ ∅ 𝜑 | 
| 13 | 12 | a1i 9 | 
. 2
⊢ ((𝐴 ∈ Fin ∧ ∀𝑥 ∈ 𝐴 DECID 𝜑) → DECID ∃𝑥 ∈ ∅ 𝜑) | 
| 14 |   | simpr 110 | 
. . . . . . . . 9
⊢
((((((𝐴 ∈ Fin
∧ ∀𝑥 ∈
𝐴 DECID
𝜑) ∧ 𝑦 ∈ Fin) ∧ (𝑦 ⊆ 𝐴 ∧ 𝑧 ∈ (𝐴 ∖ 𝑦))) ∧ DECID ∃𝑥 ∈ 𝑦 𝜑) ∧ [𝑧 / 𝑥]𝜑) → [𝑧 / 𝑥]𝜑) | 
| 15 |   | sbsbc 2993 | 
. . . . . . . . . 10
⊢ ([𝑧 / 𝑥]𝜑 ↔ [𝑧 / 𝑥]𝜑) | 
| 16 |   | rexsns 3661 | 
. . . . . . . . . 10
⊢
(∃𝑥 ∈
{𝑧}𝜑 ↔ [𝑧 / 𝑥]𝜑) | 
| 17 | 15, 16 | bitr4i 187 | 
. . . . . . . . 9
⊢ ([𝑧 / 𝑥]𝜑 ↔ ∃𝑥 ∈ {𝑧}𝜑) | 
| 18 | 14, 17 | sylib 122 | 
. . . . . . . 8
⊢
((((((𝐴 ∈ Fin
∧ ∀𝑥 ∈
𝐴 DECID
𝜑) ∧ 𝑦 ∈ Fin) ∧ (𝑦 ⊆ 𝐴 ∧ 𝑧 ∈ (𝐴 ∖ 𝑦))) ∧ DECID ∃𝑥 ∈ 𝑦 𝜑) ∧ [𝑧 / 𝑥]𝜑) → ∃𝑥 ∈ {𝑧}𝜑) | 
| 19 | 18 | olcd 735 | 
. . . . . . 7
⊢
((((((𝐴 ∈ Fin
∧ ∀𝑥 ∈
𝐴 DECID
𝜑) ∧ 𝑦 ∈ Fin) ∧ (𝑦 ⊆ 𝐴 ∧ 𝑧 ∈ (𝐴 ∖ 𝑦))) ∧ DECID ∃𝑥 ∈ 𝑦 𝜑) ∧ [𝑧 / 𝑥]𝜑) → (∃𝑥 ∈ 𝑦 𝜑 ∨ ∃𝑥 ∈ {𝑧}𝜑)) | 
| 20 |   | rexun 3343 | 
. . . . . . 7
⊢
(∃𝑥 ∈
(𝑦 ∪ {𝑧})𝜑 ↔ (∃𝑥 ∈ 𝑦 𝜑 ∨ ∃𝑥 ∈ {𝑧}𝜑)) | 
| 21 | 19, 20 | sylibr 134 | 
. . . . . 6
⊢
((((((𝐴 ∈ Fin
∧ ∀𝑥 ∈
𝐴 DECID
𝜑) ∧ 𝑦 ∈ Fin) ∧ (𝑦 ⊆ 𝐴 ∧ 𝑧 ∈ (𝐴 ∖ 𝑦))) ∧ DECID ∃𝑥 ∈ 𝑦 𝜑) ∧ [𝑧 / 𝑥]𝜑) → ∃𝑥 ∈ (𝑦 ∪ {𝑧})𝜑) | 
| 22 | 21 | orcd 734 | 
. . . . 5
⊢
((((((𝐴 ∈ Fin
∧ ∀𝑥 ∈
𝐴 DECID
𝜑) ∧ 𝑦 ∈ Fin) ∧ (𝑦 ⊆ 𝐴 ∧ 𝑧 ∈ (𝐴 ∖ 𝑦))) ∧ DECID ∃𝑥 ∈ 𝑦 𝜑) ∧ [𝑧 / 𝑥]𝜑) → (∃𝑥 ∈ (𝑦 ∪ {𝑧})𝜑 ∨ ¬ ∃𝑥 ∈ (𝑦 ∪ {𝑧})𝜑)) | 
| 23 |   | df-dc 836 | 
. . . . 5
⊢
(DECID ∃𝑥 ∈ (𝑦 ∪ {𝑧})𝜑 ↔ (∃𝑥 ∈ (𝑦 ∪ {𝑧})𝜑 ∨ ¬ ∃𝑥 ∈ (𝑦 ∪ {𝑧})𝜑)) | 
| 24 | 22, 23 | sylibr 134 | 
. . . 4
⊢
((((((𝐴 ∈ Fin
∧ ∀𝑥 ∈
𝐴 DECID
𝜑) ∧ 𝑦 ∈ Fin) ∧ (𝑦 ⊆ 𝐴 ∧ 𝑧 ∈ (𝐴 ∖ 𝑦))) ∧ DECID ∃𝑥 ∈ 𝑦 𝜑) ∧ [𝑧 / 𝑥]𝜑) → DECID ∃𝑥 ∈ (𝑦 ∪ {𝑧})𝜑) | 
| 25 |   | simpr 110 | 
. . . . . . . . 9
⊢
(((((((𝐴 ∈ Fin
∧ ∀𝑥 ∈
𝐴 DECID
𝜑) ∧ 𝑦 ∈ Fin) ∧ (𝑦 ⊆ 𝐴 ∧ 𝑧 ∈ (𝐴 ∖ 𝑦))) ∧ DECID ∃𝑥 ∈ 𝑦 𝜑) ∧ ¬ [𝑧 / 𝑥]𝜑) ∧ ∃𝑥 ∈ 𝑦 𝜑) → ∃𝑥 ∈ 𝑦 𝜑) | 
| 26 | 25 | orcd 734 | 
. . . . . . . 8
⊢
(((((((𝐴 ∈ Fin
∧ ∀𝑥 ∈
𝐴 DECID
𝜑) ∧ 𝑦 ∈ Fin) ∧ (𝑦 ⊆ 𝐴 ∧ 𝑧 ∈ (𝐴 ∖ 𝑦))) ∧ DECID ∃𝑥 ∈ 𝑦 𝜑) ∧ ¬ [𝑧 / 𝑥]𝜑) ∧ ∃𝑥 ∈ 𝑦 𝜑) → (∃𝑥 ∈ 𝑦 𝜑 ∨ ∃𝑥 ∈ {𝑧}𝜑)) | 
| 27 | 26, 20 | sylibr 134 | 
. . . . . . 7
⊢
(((((((𝐴 ∈ Fin
∧ ∀𝑥 ∈
𝐴 DECID
𝜑) ∧ 𝑦 ∈ Fin) ∧ (𝑦 ⊆ 𝐴 ∧ 𝑧 ∈ (𝐴 ∖ 𝑦))) ∧ DECID ∃𝑥 ∈ 𝑦 𝜑) ∧ ¬ [𝑧 / 𝑥]𝜑) ∧ ∃𝑥 ∈ 𝑦 𝜑) → ∃𝑥 ∈ (𝑦 ∪ {𝑧})𝜑) | 
| 28 | 27 | orcd 734 | 
. . . . . 6
⊢
(((((((𝐴 ∈ Fin
∧ ∀𝑥 ∈
𝐴 DECID
𝜑) ∧ 𝑦 ∈ Fin) ∧ (𝑦 ⊆ 𝐴 ∧ 𝑧 ∈ (𝐴 ∖ 𝑦))) ∧ DECID ∃𝑥 ∈ 𝑦 𝜑) ∧ ¬ [𝑧 / 𝑥]𝜑) ∧ ∃𝑥 ∈ 𝑦 𝜑) → (∃𝑥 ∈ (𝑦 ∪ {𝑧})𝜑 ∨ ¬ ∃𝑥 ∈ (𝑦 ∪ {𝑧})𝜑)) | 
| 29 | 28, 23 | sylibr 134 | 
. . . . 5
⊢
(((((((𝐴 ∈ Fin
∧ ∀𝑥 ∈
𝐴 DECID
𝜑) ∧ 𝑦 ∈ Fin) ∧ (𝑦 ⊆ 𝐴 ∧ 𝑧 ∈ (𝐴 ∖ 𝑦))) ∧ DECID ∃𝑥 ∈ 𝑦 𝜑) ∧ ¬ [𝑧 / 𝑥]𝜑) ∧ ∃𝑥 ∈ 𝑦 𝜑) → DECID ∃𝑥 ∈ (𝑦 ∪ {𝑧})𝜑) | 
| 30 |   | simpr 110 | 
. . . . . . . . 9
⊢
(((((((𝐴 ∈ Fin
∧ ∀𝑥 ∈
𝐴 DECID
𝜑) ∧ 𝑦 ∈ Fin) ∧ (𝑦 ⊆ 𝐴 ∧ 𝑧 ∈ (𝐴 ∖ 𝑦))) ∧ DECID ∃𝑥 ∈ 𝑦 𝜑) ∧ ¬ [𝑧 / 𝑥]𝜑) ∧ ¬ ∃𝑥 ∈ 𝑦 𝜑) → ¬ ∃𝑥 ∈ 𝑦 𝜑) | 
| 31 |   | simpr 110 | 
. . . . . . . . . . 11
⊢
((((((𝐴 ∈ Fin
∧ ∀𝑥 ∈
𝐴 DECID
𝜑) ∧ 𝑦 ∈ Fin) ∧ (𝑦 ⊆ 𝐴 ∧ 𝑧 ∈ (𝐴 ∖ 𝑦))) ∧ DECID ∃𝑥 ∈ 𝑦 𝜑) ∧ ¬ [𝑧 / 𝑥]𝜑) → ¬ [𝑧 / 𝑥]𝜑) | 
| 32 | 17 | notbii 669 | 
. . . . . . . . . . 11
⊢ (¬
[𝑧 / 𝑥]𝜑 ↔ ¬ ∃𝑥 ∈ {𝑧}𝜑) | 
| 33 | 31, 32 | sylib 122 | 
. . . . . . . . . 10
⊢
((((((𝐴 ∈ Fin
∧ ∀𝑥 ∈
𝐴 DECID
𝜑) ∧ 𝑦 ∈ Fin) ∧ (𝑦 ⊆ 𝐴 ∧ 𝑧 ∈ (𝐴 ∖ 𝑦))) ∧ DECID ∃𝑥 ∈ 𝑦 𝜑) ∧ ¬ [𝑧 / 𝑥]𝜑) → ¬ ∃𝑥 ∈ {𝑧}𝜑) | 
| 34 | 33 | adantr 276 | 
. . . . . . . . 9
⊢
(((((((𝐴 ∈ Fin
∧ ∀𝑥 ∈
𝐴 DECID
𝜑) ∧ 𝑦 ∈ Fin) ∧ (𝑦 ⊆ 𝐴 ∧ 𝑧 ∈ (𝐴 ∖ 𝑦))) ∧ DECID ∃𝑥 ∈ 𝑦 𝜑) ∧ ¬ [𝑧 / 𝑥]𝜑) ∧ ¬ ∃𝑥 ∈ 𝑦 𝜑) → ¬ ∃𝑥 ∈ {𝑧}𝜑) | 
| 35 |   | ioran 753 | 
. . . . . . . . 9
⊢ (¬
(∃𝑥 ∈ 𝑦 𝜑 ∨ ∃𝑥 ∈ {𝑧}𝜑) ↔ (¬ ∃𝑥 ∈ 𝑦 𝜑 ∧ ¬ ∃𝑥 ∈ {𝑧}𝜑)) | 
| 36 | 30, 34, 35 | sylanbrc 417 | 
. . . . . . . 8
⊢
(((((((𝐴 ∈ Fin
∧ ∀𝑥 ∈
𝐴 DECID
𝜑) ∧ 𝑦 ∈ Fin) ∧ (𝑦 ⊆ 𝐴 ∧ 𝑧 ∈ (𝐴 ∖ 𝑦))) ∧ DECID ∃𝑥 ∈ 𝑦 𝜑) ∧ ¬ [𝑧 / 𝑥]𝜑) ∧ ¬ ∃𝑥 ∈ 𝑦 𝜑) → ¬ (∃𝑥 ∈ 𝑦 𝜑 ∨ ∃𝑥 ∈ {𝑧}𝜑)) | 
| 37 | 20 | notbii 669 | 
. . . . . . . 8
⊢ (¬
∃𝑥 ∈ (𝑦 ∪ {𝑧})𝜑 ↔ ¬ (∃𝑥 ∈ 𝑦 𝜑 ∨ ∃𝑥 ∈ {𝑧}𝜑)) | 
| 38 | 36, 37 | sylibr 134 | 
. . . . . . 7
⊢
(((((((𝐴 ∈ Fin
∧ ∀𝑥 ∈
𝐴 DECID
𝜑) ∧ 𝑦 ∈ Fin) ∧ (𝑦 ⊆ 𝐴 ∧ 𝑧 ∈ (𝐴 ∖ 𝑦))) ∧ DECID ∃𝑥 ∈ 𝑦 𝜑) ∧ ¬ [𝑧 / 𝑥]𝜑) ∧ ¬ ∃𝑥 ∈ 𝑦 𝜑) → ¬ ∃𝑥 ∈ (𝑦 ∪ {𝑧})𝜑) | 
| 39 | 38 | olcd 735 | 
. . . . . 6
⊢
(((((((𝐴 ∈ Fin
∧ ∀𝑥 ∈
𝐴 DECID
𝜑) ∧ 𝑦 ∈ Fin) ∧ (𝑦 ⊆ 𝐴 ∧ 𝑧 ∈ (𝐴 ∖ 𝑦))) ∧ DECID ∃𝑥 ∈ 𝑦 𝜑) ∧ ¬ [𝑧 / 𝑥]𝜑) ∧ ¬ ∃𝑥 ∈ 𝑦 𝜑) → (∃𝑥 ∈ (𝑦 ∪ {𝑧})𝜑 ∨ ¬ ∃𝑥 ∈ (𝑦 ∪ {𝑧})𝜑)) | 
| 40 | 39, 23 | sylibr 134 | 
. . . . 5
⊢
(((((((𝐴 ∈ Fin
∧ ∀𝑥 ∈
𝐴 DECID
𝜑) ∧ 𝑦 ∈ Fin) ∧ (𝑦 ⊆ 𝐴 ∧ 𝑧 ∈ (𝐴 ∖ 𝑦))) ∧ DECID ∃𝑥 ∈ 𝑦 𝜑) ∧ ¬ [𝑧 / 𝑥]𝜑) ∧ ¬ ∃𝑥 ∈ 𝑦 𝜑) → DECID ∃𝑥 ∈ (𝑦 ∪ {𝑧})𝜑) | 
| 41 |   | exmiddc 837 | 
. . . . . 6
⊢
(DECID ∃𝑥 ∈ 𝑦 𝜑 → (∃𝑥 ∈ 𝑦 𝜑 ∨ ¬ ∃𝑥 ∈ 𝑦 𝜑)) | 
| 42 | 41 | ad2antlr 489 | 
. . . . 5
⊢
((((((𝐴 ∈ Fin
∧ ∀𝑥 ∈
𝐴 DECID
𝜑) ∧ 𝑦 ∈ Fin) ∧ (𝑦 ⊆ 𝐴 ∧ 𝑧 ∈ (𝐴 ∖ 𝑦))) ∧ DECID ∃𝑥 ∈ 𝑦 𝜑) ∧ ¬ [𝑧 / 𝑥]𝜑) → (∃𝑥 ∈ 𝑦 𝜑 ∨ ¬ ∃𝑥 ∈ 𝑦 𝜑)) | 
| 43 | 29, 40, 42 | mpjaodan 799 | 
. . . 4
⊢
((((((𝐴 ∈ Fin
∧ ∀𝑥 ∈
𝐴 DECID
𝜑) ∧ 𝑦 ∈ Fin) ∧ (𝑦 ⊆ 𝐴 ∧ 𝑧 ∈ (𝐴 ∖ 𝑦))) ∧ DECID ∃𝑥 ∈ 𝑦 𝜑) ∧ ¬ [𝑧 / 𝑥]𝜑) → DECID ∃𝑥 ∈ (𝑦 ∪ {𝑧})𝜑) | 
| 44 |   | simplrr 536 | 
. . . . . . 7
⊢
(((((𝐴 ∈ Fin
∧ ∀𝑥 ∈
𝐴 DECID
𝜑) ∧ 𝑦 ∈ Fin) ∧ (𝑦 ⊆ 𝐴 ∧ 𝑧 ∈ (𝐴 ∖ 𝑦))) ∧ DECID ∃𝑥 ∈ 𝑦 𝜑) → 𝑧 ∈ (𝐴 ∖ 𝑦)) | 
| 45 | 44 | eldifad 3168 | 
. . . . . 6
⊢
(((((𝐴 ∈ Fin
∧ ∀𝑥 ∈
𝐴 DECID
𝜑) ∧ 𝑦 ∈ Fin) ∧ (𝑦 ⊆ 𝐴 ∧ 𝑧 ∈ (𝐴 ∖ 𝑦))) ∧ DECID ∃𝑥 ∈ 𝑦 𝜑) → 𝑧 ∈ 𝐴) | 
| 46 |   | simp-4r 542 | 
. . . . . 6
⊢
(((((𝐴 ∈ Fin
∧ ∀𝑥 ∈
𝐴 DECID
𝜑) ∧ 𝑦 ∈ Fin) ∧ (𝑦 ⊆ 𝐴 ∧ 𝑧 ∈ (𝐴 ∖ 𝑦))) ∧ DECID ∃𝑥 ∈ 𝑦 𝜑) → ∀𝑥 ∈ 𝐴 DECID 𝜑) | 
| 47 |   | nfs1v 1958 | 
. . . . . . . 8
⊢
Ⅎ𝑥[𝑧 / 𝑥]𝜑 | 
| 48 | 47 | nfdc 1673 | 
. . . . . . 7
⊢
Ⅎ𝑥DECID [𝑧 / 𝑥]𝜑 | 
| 49 |   | sbequ12 1785 | 
. . . . . . . 8
⊢ (𝑥 = 𝑧 → (𝜑 ↔ [𝑧 / 𝑥]𝜑)) | 
| 50 | 49 | dcbid 839 | 
. . . . . . 7
⊢ (𝑥 = 𝑧 → (DECID 𝜑 ↔ DECID [𝑧 / 𝑥]𝜑)) | 
| 51 | 48, 50 | rspc 2862 | 
. . . . . 6
⊢ (𝑧 ∈ 𝐴 → (∀𝑥 ∈ 𝐴 DECID 𝜑 → DECID [𝑧 / 𝑥]𝜑)) | 
| 52 | 45, 46, 51 | sylc 62 | 
. . . . 5
⊢
(((((𝐴 ∈ Fin
∧ ∀𝑥 ∈
𝐴 DECID
𝜑) ∧ 𝑦 ∈ Fin) ∧ (𝑦 ⊆ 𝐴 ∧ 𝑧 ∈ (𝐴 ∖ 𝑦))) ∧ DECID ∃𝑥 ∈ 𝑦 𝜑) → DECID [𝑧 / 𝑥]𝜑) | 
| 53 |   | exmiddc 837 | 
. . . . 5
⊢
(DECID [𝑧 / 𝑥]𝜑 → ([𝑧 / 𝑥]𝜑 ∨ ¬ [𝑧 / 𝑥]𝜑)) | 
| 54 | 52, 53 | syl 14 | 
. . . 4
⊢
(((((𝐴 ∈ Fin
∧ ∀𝑥 ∈
𝐴 DECID
𝜑) ∧ 𝑦 ∈ Fin) ∧ (𝑦 ⊆ 𝐴 ∧ 𝑧 ∈ (𝐴 ∖ 𝑦))) ∧ DECID ∃𝑥 ∈ 𝑦 𝜑) → ([𝑧 / 𝑥]𝜑 ∨ ¬ [𝑧 / 𝑥]𝜑)) | 
| 55 | 24, 43, 54 | mpjaodan 799 | 
. . 3
⊢
(((((𝐴 ∈ Fin
∧ ∀𝑥 ∈
𝐴 DECID
𝜑) ∧ 𝑦 ∈ Fin) ∧ (𝑦 ⊆ 𝐴 ∧ 𝑧 ∈ (𝐴 ∖ 𝑦))) ∧ DECID ∃𝑥 ∈ 𝑦 𝜑) → DECID ∃𝑥 ∈ (𝑦 ∪ {𝑧})𝜑) | 
| 56 | 55 | ex 115 | 
. 2
⊢ ((((𝐴 ∈ Fin ∧ ∀𝑥 ∈ 𝐴 DECID 𝜑) ∧ 𝑦 ∈ Fin) ∧ (𝑦 ⊆ 𝐴 ∧ 𝑧 ∈ (𝐴 ∖ 𝑦))) → (DECID ∃𝑥 ∈ 𝑦 𝜑 → DECID ∃𝑥 ∈ (𝑦 ∪ {𝑧})𝜑)) | 
| 57 |   | simpl 109 | 
. 2
⊢ ((𝐴 ∈ Fin ∧ ∀𝑥 ∈ 𝐴 DECID 𝜑) → 𝐴 ∈ Fin) | 
| 58 | 2, 4, 6, 8, 13, 56, 57 | findcard2sd 6953 | 
1
⊢ ((𝐴 ∈ Fin ∧ ∀𝑥 ∈ 𝐴 DECID 𝜑) → DECID ∃𝑥 ∈ 𝐴 𝜑) |