Step |
Hyp |
Ref |
Expression |
1 |
|
en1 |
⊢ ( 𝐵 ≈ 1o ↔ ∃ 𝑥 𝐵 = { 𝑥 } ) |
2 |
|
eleq2 |
⊢ ( 𝐵 = { 𝑥 } → ( 𝐴 ∈ 𝐵 ↔ 𝐴 ∈ { 𝑥 } ) ) |
3 |
|
elsni |
⊢ ( 𝐴 ∈ { 𝑥 } → 𝐴 = 𝑥 ) |
4 |
3
|
sneqd |
⊢ ( 𝐴 ∈ { 𝑥 } → { 𝐴 } = { 𝑥 } ) |
5 |
2 4
|
syl6bi |
⊢ ( 𝐵 = { 𝑥 } → ( 𝐴 ∈ 𝐵 → { 𝐴 } = { 𝑥 } ) ) |
6 |
5
|
imp |
⊢ ( ( 𝐵 = { 𝑥 } ∧ 𝐴 ∈ 𝐵 ) → { 𝐴 } = { 𝑥 } ) |
7 |
|
eqtr3 |
⊢ ( ( 𝐵 = { 𝑥 } ∧ { 𝐴 } = { 𝑥 } ) → 𝐵 = { 𝐴 } ) |
8 |
6 7
|
syldan |
⊢ ( ( 𝐵 = { 𝑥 } ∧ 𝐴 ∈ 𝐵 ) → 𝐵 = { 𝐴 } ) |
9 |
8
|
ex |
⊢ ( 𝐵 = { 𝑥 } → ( 𝐴 ∈ 𝐵 → 𝐵 = { 𝐴 } ) ) |
10 |
9
|
exlimiv |
⊢ ( ∃ 𝑥 𝐵 = { 𝑥 } → ( 𝐴 ∈ 𝐵 → 𝐵 = { 𝐴 } ) ) |
11 |
1 10
|
sylbi |
⊢ ( 𝐵 ≈ 1o → ( 𝐴 ∈ 𝐵 → 𝐵 = { 𝐴 } ) ) |
12 |
11
|
impcom |
⊢ ( ( 𝐴 ∈ 𝐵 ∧ 𝐵 ≈ 1o ) → 𝐵 = { 𝐴 } ) |