| Step | Hyp | Ref
 | Expression | 
| 1 |   | relco 5168 | 
. 2
⊢ Rel
(𝐴 ∘ ∪ 𝑥 ∈ 𝐶 𝐵) | 
| 2 |   | reliun 4784 | 
. . 3
⊢ (Rel
∪ 𝑥 ∈ 𝐶 (𝐴 ∘ 𝐵) ↔ ∀𝑥 ∈ 𝐶 Rel (𝐴 ∘ 𝐵)) | 
| 3 |   | relco 5168 | 
. . . 4
⊢ Rel
(𝐴 ∘ 𝐵) | 
| 4 | 3 | a1i 9 | 
. . 3
⊢ (𝑥 ∈ 𝐶 → Rel (𝐴 ∘ 𝐵)) | 
| 5 | 2, 4 | mprgbir 2555 | 
. 2
⊢ Rel
∪ 𝑥 ∈ 𝐶 (𝐴 ∘ 𝐵) | 
| 6 |   | eliun 3920 | 
. . . . . . . 8
⊢
(〈𝑦, 𝑤〉 ∈ ∪ 𝑥 ∈ 𝐶 𝐵 ↔ ∃𝑥 ∈ 𝐶 〈𝑦, 𝑤〉 ∈ 𝐵) | 
| 7 |   | df-br 4034 | 
. . . . . . . 8
⊢ (𝑦∪ 𝑥 ∈ 𝐶 𝐵𝑤 ↔ 〈𝑦, 𝑤〉 ∈ ∪ 𝑥 ∈ 𝐶 𝐵) | 
| 8 |   | df-br 4034 | 
. . . . . . . . 9
⊢ (𝑦𝐵𝑤 ↔ 〈𝑦, 𝑤〉 ∈ 𝐵) | 
| 9 | 8 | rexbii 2504 | 
. . . . . . . 8
⊢
(∃𝑥 ∈
𝐶 𝑦𝐵𝑤 ↔ ∃𝑥 ∈ 𝐶 〈𝑦, 𝑤〉 ∈ 𝐵) | 
| 10 | 6, 7, 9 | 3bitr4i 212 | 
. . . . . . 7
⊢ (𝑦∪ 𝑥 ∈ 𝐶 𝐵𝑤 ↔ ∃𝑥 ∈ 𝐶 𝑦𝐵𝑤) | 
| 11 | 10 | anbi1i 458 | 
. . . . . 6
⊢ ((𝑦∪ 𝑥 ∈ 𝐶 𝐵𝑤 ∧ 𝑤𝐴𝑧) ↔ (∃𝑥 ∈ 𝐶 𝑦𝐵𝑤 ∧ 𝑤𝐴𝑧)) | 
| 12 |   | r19.41v 2653 | 
. . . . . 6
⊢
(∃𝑥 ∈
𝐶 (𝑦𝐵𝑤 ∧ 𝑤𝐴𝑧) ↔ (∃𝑥 ∈ 𝐶 𝑦𝐵𝑤 ∧ 𝑤𝐴𝑧)) | 
| 13 | 11, 12 | bitr4i 187 | 
. . . . 5
⊢ ((𝑦∪ 𝑥 ∈ 𝐶 𝐵𝑤 ∧ 𝑤𝐴𝑧) ↔ ∃𝑥 ∈ 𝐶 (𝑦𝐵𝑤 ∧ 𝑤𝐴𝑧)) | 
| 14 | 13 | exbii 1619 | 
. . . 4
⊢
(∃𝑤(𝑦∪ 𝑥 ∈ 𝐶 𝐵𝑤 ∧ 𝑤𝐴𝑧) ↔ ∃𝑤∃𝑥 ∈ 𝐶 (𝑦𝐵𝑤 ∧ 𝑤𝐴𝑧)) | 
| 15 |   | rexcom4 2786 | 
. . . 4
⊢
(∃𝑥 ∈
𝐶 ∃𝑤(𝑦𝐵𝑤 ∧ 𝑤𝐴𝑧) ↔ ∃𝑤∃𝑥 ∈ 𝐶 (𝑦𝐵𝑤 ∧ 𝑤𝐴𝑧)) | 
| 16 | 14, 15 | bitr4i 187 | 
. . 3
⊢
(∃𝑤(𝑦∪ 𝑥 ∈ 𝐶 𝐵𝑤 ∧ 𝑤𝐴𝑧) ↔ ∃𝑥 ∈ 𝐶 ∃𝑤(𝑦𝐵𝑤 ∧ 𝑤𝐴𝑧)) | 
| 17 |   | vex 2766 | 
. . . 4
⊢ 𝑦 ∈ V | 
| 18 |   | vex 2766 | 
. . . 4
⊢ 𝑧 ∈ V | 
| 19 | 17, 18 | opelco 4838 | 
. . 3
⊢
(〈𝑦, 𝑧〉 ∈ (𝐴 ∘ ∪
𝑥 ∈ 𝐶 𝐵) ↔ ∃𝑤(𝑦∪ 𝑥 ∈ 𝐶 𝐵𝑤 ∧ 𝑤𝐴𝑧)) | 
| 20 |   | eliun 3920 | 
. . . 4
⊢
(〈𝑦, 𝑧〉 ∈ ∪ 𝑥 ∈ 𝐶 (𝐴 ∘ 𝐵) ↔ ∃𝑥 ∈ 𝐶 〈𝑦, 𝑧〉 ∈ (𝐴 ∘ 𝐵)) | 
| 21 | 17, 18 | opelco 4838 | 
. . . . 5
⊢
(〈𝑦, 𝑧〉 ∈ (𝐴 ∘ 𝐵) ↔ ∃𝑤(𝑦𝐵𝑤 ∧ 𝑤𝐴𝑧)) | 
| 22 | 21 | rexbii 2504 | 
. . . 4
⊢
(∃𝑥 ∈
𝐶 〈𝑦, 𝑧〉 ∈ (𝐴 ∘ 𝐵) ↔ ∃𝑥 ∈ 𝐶 ∃𝑤(𝑦𝐵𝑤 ∧ 𝑤𝐴𝑧)) | 
| 23 | 20, 22 | bitri 184 | 
. . 3
⊢
(〈𝑦, 𝑧〉 ∈ ∪ 𝑥 ∈ 𝐶 (𝐴 ∘ 𝐵) ↔ ∃𝑥 ∈ 𝐶 ∃𝑤(𝑦𝐵𝑤 ∧ 𝑤𝐴𝑧)) | 
| 24 | 16, 19, 23 | 3bitr4i 212 | 
. 2
⊢
(〈𝑦, 𝑧〉 ∈ (𝐴 ∘ ∪
𝑥 ∈ 𝐶 𝐵) ↔ 〈𝑦, 𝑧〉 ∈ ∪ 𝑥 ∈ 𝐶 (𝐴 ∘ 𝐵)) | 
| 25 | 1, 5, 24 | eqrelriiv 4757 | 
1
⊢ (𝐴 ∘ ∪ 𝑥 ∈ 𝐶 𝐵) = ∪
𝑥 ∈ 𝐶 (𝐴 ∘ 𝐵) |