| Step | Hyp | Ref | Expression | 
						
							| 1 |  | idn1 | ⊢ (    𝐴  ∈  𝐵    ▶    𝐴  ∈  𝐵    ) | 
						
							| 2 |  | sbcel2 | ⊢ ( [ 𝐴  /  𝑥 ] 𝑦  ∈  𝐶  ↔  𝑦  ∈  ⦋ 𝐴  /  𝑥 ⦌ 𝐶 ) | 
						
							| 3 | 2 | a1i | ⊢ ( 𝐴  ∈  𝐵  →  ( [ 𝐴  /  𝑥 ] 𝑦  ∈  𝐶  ↔  𝑦  ∈  ⦋ 𝐴  /  𝑥 ⦌ 𝐶 ) ) | 
						
							| 4 | 1 3 | e1a | ⊢ (    𝐴  ∈  𝐵    ▶    ( [ 𝐴  /  𝑥 ] 𝑦  ∈  𝐶  ↔  𝑦  ∈  ⦋ 𝐴  /  𝑥 ⦌ 𝐶 )    ) | 
						
							| 5 |  | sbcel2 | ⊢ ( [ 𝐴  /  𝑥 ] 𝑦  ∈  𝐷  ↔  𝑦  ∈  ⦋ 𝐴  /  𝑥 ⦌ 𝐷 ) | 
						
							| 6 | 5 | a1i | ⊢ ( 𝐴  ∈  𝐵  →  ( [ 𝐴  /  𝑥 ] 𝑦  ∈  𝐷  ↔  𝑦  ∈  ⦋ 𝐴  /  𝑥 ⦌ 𝐷 ) ) | 
						
							| 7 | 1 6 | e1a | ⊢ (    𝐴  ∈  𝐵    ▶    ( [ 𝐴  /  𝑥 ] 𝑦  ∈  𝐷  ↔  𝑦  ∈  ⦋ 𝐴  /  𝑥 ⦌ 𝐷 )    ) | 
						
							| 8 |  | imbi12 | ⊢ ( ( [ 𝐴  /  𝑥 ] 𝑦  ∈  𝐶  ↔  𝑦  ∈  ⦋ 𝐴  /  𝑥 ⦌ 𝐶 )  →  ( ( [ 𝐴  /  𝑥 ] 𝑦  ∈  𝐷  ↔  𝑦  ∈  ⦋ 𝐴  /  𝑥 ⦌ 𝐷 )  →  ( ( [ 𝐴  /  𝑥 ] 𝑦  ∈  𝐶  →  [ 𝐴  /  𝑥 ] 𝑦  ∈  𝐷 )  ↔  ( 𝑦  ∈  ⦋ 𝐴  /  𝑥 ⦌ 𝐶  →  𝑦  ∈  ⦋ 𝐴  /  𝑥 ⦌ 𝐷 ) ) ) ) | 
						
							| 9 | 4 7 8 | e11 | ⊢ (    𝐴  ∈  𝐵    ▶    ( ( [ 𝐴  /  𝑥 ] 𝑦  ∈  𝐶  →  [ 𝐴  /  𝑥 ] 𝑦  ∈  𝐷 )  ↔  ( 𝑦  ∈  ⦋ 𝐴  /  𝑥 ⦌ 𝐶  →  𝑦  ∈  ⦋ 𝐴  /  𝑥 ⦌ 𝐷 ) )    ) | 
						
							| 10 |  | sbcimg | ⊢ ( 𝐴  ∈  𝐵  →  ( [ 𝐴  /  𝑥 ] ( 𝑦  ∈  𝐶  →  𝑦  ∈  𝐷 )  ↔  ( [ 𝐴  /  𝑥 ] 𝑦  ∈  𝐶  →  [ 𝐴  /  𝑥 ] 𝑦  ∈  𝐷 ) ) ) | 
						
							| 11 | 1 10 | e1a | ⊢ (    𝐴  ∈  𝐵    ▶    ( [ 𝐴  /  𝑥 ] ( 𝑦  ∈  𝐶  →  𝑦  ∈  𝐷 )  ↔  ( [ 𝐴  /  𝑥 ] 𝑦  ∈  𝐶  →  [ 𝐴  /  𝑥 ] 𝑦  ∈  𝐷 ) )    ) | 
						
							| 12 |  | bibi1 | ⊢ ( ( [ 𝐴  /  𝑥 ] ( 𝑦  ∈  𝐶  →  𝑦  ∈  𝐷 )  ↔  ( [ 𝐴  /  𝑥 ] 𝑦  ∈  𝐶  →  [ 𝐴  /  𝑥 ] 𝑦  ∈  𝐷 ) )  →  ( ( [ 𝐴  /  𝑥 ] ( 𝑦  ∈  𝐶  →  𝑦  ∈  𝐷 )  ↔  ( 𝑦  ∈  ⦋ 𝐴  /  𝑥 ⦌ 𝐶  →  𝑦  ∈  ⦋ 𝐴  /  𝑥 ⦌ 𝐷 ) )  ↔  ( ( [ 𝐴  /  𝑥 ] 𝑦  ∈  𝐶  →  [ 𝐴  /  𝑥 ] 𝑦  ∈  𝐷 )  ↔  ( 𝑦  ∈  ⦋ 𝐴  /  𝑥 ⦌ 𝐶  →  𝑦  ∈  ⦋ 𝐴  /  𝑥 ⦌ 𝐷 ) ) ) ) | 
						
							| 13 | 12 | biimprcd | ⊢ ( ( ( [ 𝐴  /  𝑥 ] 𝑦  ∈  𝐶  →  [ 𝐴  /  𝑥 ] 𝑦  ∈  𝐷 )  ↔  ( 𝑦  ∈  ⦋ 𝐴  /  𝑥 ⦌ 𝐶  →  𝑦  ∈  ⦋ 𝐴  /  𝑥 ⦌ 𝐷 ) )  →  ( ( [ 𝐴  /  𝑥 ] ( 𝑦  ∈  𝐶  →  𝑦  ∈  𝐷 )  ↔  ( [ 𝐴  /  𝑥 ] 𝑦  ∈  𝐶  →  [ 𝐴  /  𝑥 ] 𝑦  ∈  𝐷 ) )  →  ( [ 𝐴  /  𝑥 ] ( 𝑦  ∈  𝐶  →  𝑦  ∈  𝐷 )  ↔  ( 𝑦  ∈  ⦋ 𝐴  /  𝑥 ⦌ 𝐶  →  𝑦  ∈  ⦋ 𝐴  /  𝑥 ⦌ 𝐷 ) ) ) ) | 
						
							| 14 | 9 11 13 | e11 | ⊢ (    𝐴  ∈  𝐵    ▶    ( [ 𝐴  /  𝑥 ] ( 𝑦  ∈  𝐶  →  𝑦  ∈  𝐷 )  ↔  ( 𝑦  ∈  ⦋ 𝐴  /  𝑥 ⦌ 𝐶  →  𝑦  ∈  ⦋ 𝐴  /  𝑥 ⦌ 𝐷 ) )    ) | 
						
							| 15 | 14 | gen11 | ⊢ (    𝐴  ∈  𝐵    ▶    ∀ 𝑦 ( [ 𝐴  /  𝑥 ] ( 𝑦  ∈  𝐶  →  𝑦  ∈  𝐷 )  ↔  ( 𝑦  ∈  ⦋ 𝐴  /  𝑥 ⦌ 𝐶  →  𝑦  ∈  ⦋ 𝐴  /  𝑥 ⦌ 𝐷 ) )    ) | 
						
							| 16 |  | albi | ⊢ ( ∀ 𝑦 ( [ 𝐴  /  𝑥 ] ( 𝑦  ∈  𝐶  →  𝑦  ∈  𝐷 )  ↔  ( 𝑦  ∈  ⦋ 𝐴  /  𝑥 ⦌ 𝐶  →  𝑦  ∈  ⦋ 𝐴  /  𝑥 ⦌ 𝐷 ) )  →  ( ∀ 𝑦 [ 𝐴  /  𝑥 ] ( 𝑦  ∈  𝐶  →  𝑦  ∈  𝐷 )  ↔  ∀ 𝑦 ( 𝑦  ∈  ⦋ 𝐴  /  𝑥 ⦌ 𝐶  →  𝑦  ∈  ⦋ 𝐴  /  𝑥 ⦌ 𝐷 ) ) ) | 
						
							| 17 | 15 16 | e1a | ⊢ (    𝐴  ∈  𝐵    ▶    ( ∀ 𝑦 [ 𝐴  /  𝑥 ] ( 𝑦  ∈  𝐶  →  𝑦  ∈  𝐷 )  ↔  ∀ 𝑦 ( 𝑦  ∈  ⦋ 𝐴  /  𝑥 ⦌ 𝐶  →  𝑦  ∈  ⦋ 𝐴  /  𝑥 ⦌ 𝐷 ) )    ) | 
						
							| 18 |  | sbcal | ⊢ ( [ 𝐴  /  𝑥 ] ∀ 𝑦 ( 𝑦  ∈  𝐶  →  𝑦  ∈  𝐷 )  ↔  ∀ 𝑦 [ 𝐴  /  𝑥 ] ( 𝑦  ∈  𝐶  →  𝑦  ∈  𝐷 ) ) | 
						
							| 19 | 18 | a1i | ⊢ ( 𝐴  ∈  𝐵  →  ( [ 𝐴  /  𝑥 ] ∀ 𝑦 ( 𝑦  ∈  𝐶  →  𝑦  ∈  𝐷 )  ↔  ∀ 𝑦 [ 𝐴  /  𝑥 ] ( 𝑦  ∈  𝐶  →  𝑦  ∈  𝐷 ) ) ) | 
						
							| 20 | 1 19 | e1a | ⊢ (    𝐴  ∈  𝐵    ▶    ( [ 𝐴  /  𝑥 ] ∀ 𝑦 ( 𝑦  ∈  𝐶  →  𝑦  ∈  𝐷 )  ↔  ∀ 𝑦 [ 𝐴  /  𝑥 ] ( 𝑦  ∈  𝐶  →  𝑦  ∈  𝐷 ) )    ) | 
						
							| 21 |  | bibi1 | ⊢ ( ( [ 𝐴  /  𝑥 ] ∀ 𝑦 ( 𝑦  ∈  𝐶  →  𝑦  ∈  𝐷 )  ↔  ∀ 𝑦 [ 𝐴  /  𝑥 ] ( 𝑦  ∈  𝐶  →  𝑦  ∈  𝐷 ) )  →  ( ( [ 𝐴  /  𝑥 ] ∀ 𝑦 ( 𝑦  ∈  𝐶  →  𝑦  ∈  𝐷 )  ↔  ∀ 𝑦 ( 𝑦  ∈  ⦋ 𝐴  /  𝑥 ⦌ 𝐶  →  𝑦  ∈  ⦋ 𝐴  /  𝑥 ⦌ 𝐷 ) )  ↔  ( ∀ 𝑦 [ 𝐴  /  𝑥 ] ( 𝑦  ∈  𝐶  →  𝑦  ∈  𝐷 )  ↔  ∀ 𝑦 ( 𝑦  ∈  ⦋ 𝐴  /  𝑥 ⦌ 𝐶  →  𝑦  ∈  ⦋ 𝐴  /  𝑥 ⦌ 𝐷 ) ) ) ) | 
						
							| 22 | 21 | biimprcd | ⊢ ( ( ∀ 𝑦 [ 𝐴  /  𝑥 ] ( 𝑦  ∈  𝐶  →  𝑦  ∈  𝐷 )  ↔  ∀ 𝑦 ( 𝑦  ∈  ⦋ 𝐴  /  𝑥 ⦌ 𝐶  →  𝑦  ∈  ⦋ 𝐴  /  𝑥 ⦌ 𝐷 ) )  →  ( ( [ 𝐴  /  𝑥 ] ∀ 𝑦 ( 𝑦  ∈  𝐶  →  𝑦  ∈  𝐷 )  ↔  ∀ 𝑦 [ 𝐴  /  𝑥 ] ( 𝑦  ∈  𝐶  →  𝑦  ∈  𝐷 ) )  →  ( [ 𝐴  /  𝑥 ] ∀ 𝑦 ( 𝑦  ∈  𝐶  →  𝑦  ∈  𝐷 )  ↔  ∀ 𝑦 ( 𝑦  ∈  ⦋ 𝐴  /  𝑥 ⦌ 𝐶  →  𝑦  ∈  ⦋ 𝐴  /  𝑥 ⦌ 𝐷 ) ) ) ) | 
						
							| 23 | 17 20 22 | e11 | ⊢ (    𝐴  ∈  𝐵    ▶    ( [ 𝐴  /  𝑥 ] ∀ 𝑦 ( 𝑦  ∈  𝐶  →  𝑦  ∈  𝐷 )  ↔  ∀ 𝑦 ( 𝑦  ∈  ⦋ 𝐴  /  𝑥 ⦌ 𝐶  →  𝑦  ∈  ⦋ 𝐴  /  𝑥 ⦌ 𝐷 ) )    ) | 
						
							| 24 |  | df-ss | ⊢ ( 𝐶  ⊆  𝐷  ↔  ∀ 𝑦 ( 𝑦  ∈  𝐶  →  𝑦  ∈  𝐷 ) ) | 
						
							| 25 | 24 | ax-gen | ⊢ ∀ 𝑥 ( 𝐶  ⊆  𝐷  ↔  ∀ 𝑦 ( 𝑦  ∈  𝐶  →  𝑦  ∈  𝐷 ) ) | 
						
							| 26 |  | sbcbi | ⊢ ( 𝐴  ∈  𝐵  →  ( ∀ 𝑥 ( 𝐶  ⊆  𝐷  ↔  ∀ 𝑦 ( 𝑦  ∈  𝐶  →  𝑦  ∈  𝐷 ) )  →  ( [ 𝐴  /  𝑥 ] 𝐶  ⊆  𝐷  ↔  [ 𝐴  /  𝑥 ] ∀ 𝑦 ( 𝑦  ∈  𝐶  →  𝑦  ∈  𝐷 ) ) ) ) | 
						
							| 27 | 1 25 26 | e10 | ⊢ (    𝐴  ∈  𝐵    ▶    ( [ 𝐴  /  𝑥 ] 𝐶  ⊆  𝐷  ↔  [ 𝐴  /  𝑥 ] ∀ 𝑦 ( 𝑦  ∈  𝐶  →  𝑦  ∈  𝐷 ) )    ) | 
						
							| 28 |  | bibi1 | ⊢ ( ( [ 𝐴  /  𝑥 ] 𝐶  ⊆  𝐷  ↔  [ 𝐴  /  𝑥 ] ∀ 𝑦 ( 𝑦  ∈  𝐶  →  𝑦  ∈  𝐷 ) )  →  ( ( [ 𝐴  /  𝑥 ] 𝐶  ⊆  𝐷  ↔  ∀ 𝑦 ( 𝑦  ∈  ⦋ 𝐴  /  𝑥 ⦌ 𝐶  →  𝑦  ∈  ⦋ 𝐴  /  𝑥 ⦌ 𝐷 ) )  ↔  ( [ 𝐴  /  𝑥 ] ∀ 𝑦 ( 𝑦  ∈  𝐶  →  𝑦  ∈  𝐷 )  ↔  ∀ 𝑦 ( 𝑦  ∈  ⦋ 𝐴  /  𝑥 ⦌ 𝐶  →  𝑦  ∈  ⦋ 𝐴  /  𝑥 ⦌ 𝐷 ) ) ) ) | 
						
							| 29 | 28 | biimprcd | ⊢ ( ( [ 𝐴  /  𝑥 ] ∀ 𝑦 ( 𝑦  ∈  𝐶  →  𝑦  ∈  𝐷 )  ↔  ∀ 𝑦 ( 𝑦  ∈  ⦋ 𝐴  /  𝑥 ⦌ 𝐶  →  𝑦  ∈  ⦋ 𝐴  /  𝑥 ⦌ 𝐷 ) )  →  ( ( [ 𝐴  /  𝑥 ] 𝐶  ⊆  𝐷  ↔  [ 𝐴  /  𝑥 ] ∀ 𝑦 ( 𝑦  ∈  𝐶  →  𝑦  ∈  𝐷 ) )  →  ( [ 𝐴  /  𝑥 ] 𝐶  ⊆  𝐷  ↔  ∀ 𝑦 ( 𝑦  ∈  ⦋ 𝐴  /  𝑥 ⦌ 𝐶  →  𝑦  ∈  ⦋ 𝐴  /  𝑥 ⦌ 𝐷 ) ) ) ) | 
						
							| 30 | 23 27 29 | e11 | ⊢ (    𝐴  ∈  𝐵    ▶    ( [ 𝐴  /  𝑥 ] 𝐶  ⊆  𝐷  ↔  ∀ 𝑦 ( 𝑦  ∈  ⦋ 𝐴  /  𝑥 ⦌ 𝐶  →  𝑦  ∈  ⦋ 𝐴  /  𝑥 ⦌ 𝐷 ) )    ) | 
						
							| 31 |  | df-ss | ⊢ ( ⦋ 𝐴  /  𝑥 ⦌ 𝐶  ⊆  ⦋ 𝐴  /  𝑥 ⦌ 𝐷  ↔  ∀ 𝑦 ( 𝑦  ∈  ⦋ 𝐴  /  𝑥 ⦌ 𝐶  →  𝑦  ∈  ⦋ 𝐴  /  𝑥 ⦌ 𝐷 ) ) | 
						
							| 32 |  | biantr | ⊢ ( ( ( [ 𝐴  /  𝑥 ] 𝐶  ⊆  𝐷  ↔  ∀ 𝑦 ( 𝑦  ∈  ⦋ 𝐴  /  𝑥 ⦌ 𝐶  →  𝑦  ∈  ⦋ 𝐴  /  𝑥 ⦌ 𝐷 ) )  ∧  ( ⦋ 𝐴  /  𝑥 ⦌ 𝐶  ⊆  ⦋ 𝐴  /  𝑥 ⦌ 𝐷  ↔  ∀ 𝑦 ( 𝑦  ∈  ⦋ 𝐴  /  𝑥 ⦌ 𝐶  →  𝑦  ∈  ⦋ 𝐴  /  𝑥 ⦌ 𝐷 ) ) )  →  ( [ 𝐴  /  𝑥 ] 𝐶  ⊆  𝐷  ↔  ⦋ 𝐴  /  𝑥 ⦌ 𝐶  ⊆  ⦋ 𝐴  /  𝑥 ⦌ 𝐷 ) ) | 
						
							| 33 | 32 | ex | ⊢ ( ( [ 𝐴  /  𝑥 ] 𝐶  ⊆  𝐷  ↔  ∀ 𝑦 ( 𝑦  ∈  ⦋ 𝐴  /  𝑥 ⦌ 𝐶  →  𝑦  ∈  ⦋ 𝐴  /  𝑥 ⦌ 𝐷 ) )  →  ( ( ⦋ 𝐴  /  𝑥 ⦌ 𝐶  ⊆  ⦋ 𝐴  /  𝑥 ⦌ 𝐷  ↔  ∀ 𝑦 ( 𝑦  ∈  ⦋ 𝐴  /  𝑥 ⦌ 𝐶  →  𝑦  ∈  ⦋ 𝐴  /  𝑥 ⦌ 𝐷 ) )  →  ( [ 𝐴  /  𝑥 ] 𝐶  ⊆  𝐷  ↔  ⦋ 𝐴  /  𝑥 ⦌ 𝐶  ⊆  ⦋ 𝐴  /  𝑥 ⦌ 𝐷 ) ) ) | 
						
							| 34 | 30 31 33 | e10 | ⊢ (    𝐴  ∈  𝐵    ▶    ( [ 𝐴  /  𝑥 ] 𝐶  ⊆  𝐷  ↔  ⦋ 𝐴  /  𝑥 ⦌ 𝐶  ⊆  ⦋ 𝐴  /  𝑥 ⦌ 𝐷 )    ) | 
						
							| 35 | 34 | in1 | ⊢ ( 𝐴  ∈  𝐵  →  ( [ 𝐴  /  𝑥 ] 𝐶  ⊆  𝐷  ↔  ⦋ 𝐴  /  𝑥 ⦌ 𝐶  ⊆  ⦋ 𝐴  /  𝑥 ⦌ 𝐷 ) ) |