Step |
Hyp |
Ref |
Expression |
1 |
|
sbcexf.1 |
⊢ Ⅎ 𝑦 𝐴 |
2 |
|
nfv |
⊢ Ⅎ 𝑧 𝜑 |
3 |
2
|
sb8ev |
⊢ ( ∃ 𝑦 𝜑 ↔ ∃ 𝑧 [ 𝑧 / 𝑦 ] 𝜑 ) |
4 |
3
|
sbcbii |
⊢ ( [ 𝐴 / 𝑥 ] ∃ 𝑦 𝜑 ↔ [ 𝐴 / 𝑥 ] ∃ 𝑧 [ 𝑧 / 𝑦 ] 𝜑 ) |
5 |
|
sbcex2 |
⊢ ( [ 𝐴 / 𝑥 ] ∃ 𝑧 [ 𝑧 / 𝑦 ] 𝜑 ↔ ∃ 𝑧 [ 𝐴 / 𝑥 ] [ 𝑧 / 𝑦 ] 𝜑 ) |
6 |
|
nfs1v |
⊢ Ⅎ 𝑦 [ 𝑧 / 𝑦 ] 𝜑 |
7 |
1 6
|
nfsbcw |
⊢ Ⅎ 𝑦 [ 𝐴 / 𝑥 ] [ 𝑧 / 𝑦 ] 𝜑 |
8 |
|
nfv |
⊢ Ⅎ 𝑧 [ 𝐴 / 𝑥 ] 𝜑 |
9 |
|
sbequ12r |
⊢ ( 𝑧 = 𝑦 → ( [ 𝑧 / 𝑦 ] 𝜑 ↔ 𝜑 ) ) |
10 |
9
|
sbcbidv |
⊢ ( 𝑧 = 𝑦 → ( [ 𝐴 / 𝑥 ] [ 𝑧 / 𝑦 ] 𝜑 ↔ [ 𝐴 / 𝑥 ] 𝜑 ) ) |
11 |
7 8 10
|
cbvexv1 |
⊢ ( ∃ 𝑧 [ 𝐴 / 𝑥 ] [ 𝑧 / 𝑦 ] 𝜑 ↔ ∃ 𝑦 [ 𝐴 / 𝑥 ] 𝜑 ) |
12 |
4 5 11
|
3bitri |
⊢ ( [ 𝐴 / 𝑥 ] ∃ 𝑦 𝜑 ↔ ∃ 𝑦 [ 𝐴 / 𝑥 ] 𝜑 ) |