Detailed syntax breakdown of Definition df-op
| Step | Hyp | Ref
 | Expression | 
| 1 |   | cA | 
. . 3
class A | 
| 2 |   | cB | 
. . 3
class B | 
| 3 | 1, 2 | cop 4562 | 
. 2
class 〈A, B〉 | 
| 4 |   | vx | 
. . . . . . 7
setvar x | 
| 5 | 4 | cv 1641 | 
. . . . . 6
class x | 
| 6 |   | vy | 
. . . . . . . 8
setvar y | 
| 7 | 6 | cv 1641 | 
. . . . . . 7
class y | 
| 8 | 7 | cphi 4563 | 
. . . . . 6
class  Phi
y | 
| 9 | 5, 8 | wceq 1642 | 
. . . . 5
wff x =
 Phi y | 
| 10 | 9, 6, 1 | wrex 2616 | 
. . . 4
wff ∃y ∈ A x =  Phi y | 
| 11 | 10, 4 | cab 2339 | 
. . 3
class {x ∣ ∃y ∈ A x =  Phi y} | 
| 12 |   | c0c 4375 | 
. . . . . . . 8
class 0c | 
| 13 | 12 | csn 3738 | 
. . . . . . 7
class
{0c} | 
| 14 | 8, 13 | cun 3208 | 
. . . . . 6
class ( Phi
y ∪
{0c}) | 
| 15 | 5, 14 | wceq 1642 | 
. . . . 5
wff x =
( Phi y ∪
{0c}) | 
| 16 | 15, 6, 2 | wrex 2616 | 
. . . 4
wff ∃y ∈ B x = ( Phi y ∪ {0c}) | 
| 17 | 16, 4 | cab 2339 | 
. . 3
class {x ∣ ∃y ∈ B x = ( Phi y ∪ {0c})} | 
| 18 | 11, 17 | cun 3208 | 
. 2
class ({x ∣ ∃y ∈ A x =  Phi y} ∪ {x
∣ ∃y ∈ B x = ( Phi y ∪ {0c})}) | 
| 19 | 3, 18 | wceq 1642 | 
1
wff 〈A, B〉 = ({x ∣ ∃y ∈ A x =  Phi y} ∪ {x
∣ ∃y ∈ B x = ( Phi y ∪ {0c})}) |