| Step | Hyp | Ref | Expression | 
						
							| 1 |  | elfm2.l | ⊢ 𝐿  =  ( 𝑌 filGen 𝐵 ) | 
						
							| 2 |  | foima | ⊢ ( 𝐹 : 𝑌 –onto→ 𝑋  →  ( 𝐹  “  𝑌 )  =  𝑋 ) | 
						
							| 3 | 2 | adantl | ⊢ ( ( 𝐵  ∈  ( fBas ‘ 𝑌 )  ∧  𝐹 : 𝑌 –onto→ 𝑋 )  →  ( 𝐹  “  𝑌 )  =  𝑋 ) | 
						
							| 4 |  | fofun | ⊢ ( 𝐹 : 𝑌 –onto→ 𝑋  →  Fun  𝐹 ) | 
						
							| 5 |  | elfvdm | ⊢ ( 𝐵  ∈  ( fBas ‘ 𝑌 )  →  𝑌  ∈  dom  fBas ) | 
						
							| 6 |  | funimaexg | ⊢ ( ( Fun  𝐹  ∧  𝑌  ∈  dom  fBas )  →  ( 𝐹  “  𝑌 )  ∈  V ) | 
						
							| 7 | 4 5 6 | syl2anr | ⊢ ( ( 𝐵  ∈  ( fBas ‘ 𝑌 )  ∧  𝐹 : 𝑌 –onto→ 𝑋 )  →  ( 𝐹  “  𝑌 )  ∈  V ) | 
						
							| 8 | 3 7 | eqeltrrd | ⊢ ( ( 𝐵  ∈  ( fBas ‘ 𝑌 )  ∧  𝐹 : 𝑌 –onto→ 𝑋 )  →  𝑋  ∈  V ) | 
						
							| 9 |  | fof | ⊢ ( 𝐹 : 𝑌 –onto→ 𝑋  →  𝐹 : 𝑌 ⟶ 𝑋 ) | 
						
							| 10 | 1 | elfm2 | ⊢ ( ( 𝑋  ∈  V  ∧  𝐵  ∈  ( fBas ‘ 𝑌 )  ∧  𝐹 : 𝑌 ⟶ 𝑋 )  →  ( 𝐴  ∈  ( ( 𝑋  FilMap  𝐹 ) ‘ 𝐵 )  ↔  ( 𝐴  ⊆  𝑋  ∧  ∃ 𝑦  ∈  𝐿 ( 𝐹  “  𝑦 )  ⊆  𝐴 ) ) ) | 
						
							| 11 | 9 10 | syl3an3 | ⊢ ( ( 𝑋  ∈  V  ∧  𝐵  ∈  ( fBas ‘ 𝑌 )  ∧  𝐹 : 𝑌 –onto→ 𝑋 )  →  ( 𝐴  ∈  ( ( 𝑋  FilMap  𝐹 ) ‘ 𝐵 )  ↔  ( 𝐴  ⊆  𝑋  ∧  ∃ 𝑦  ∈  𝐿 ( 𝐹  “  𝑦 )  ⊆  𝐴 ) ) ) | 
						
							| 12 |  | fgcl | ⊢ ( 𝐵  ∈  ( fBas ‘ 𝑌 )  →  ( 𝑌 filGen 𝐵 )  ∈  ( Fil ‘ 𝑌 ) ) | 
						
							| 13 | 1 12 | eqeltrid | ⊢ ( 𝐵  ∈  ( fBas ‘ 𝑌 )  →  𝐿  ∈  ( Fil ‘ 𝑌 ) ) | 
						
							| 14 | 13 | 3ad2ant2 | ⊢ ( ( 𝑋  ∈  V  ∧  𝐵  ∈  ( fBas ‘ 𝑌 )  ∧  𝐹 : 𝑌 –onto→ 𝑋 )  →  𝐿  ∈  ( Fil ‘ 𝑌 ) ) | 
						
							| 15 | 14 | ad2antrr | ⊢ ( ( ( ( 𝑋  ∈  V  ∧  𝐵  ∈  ( fBas ‘ 𝑌 )  ∧  𝐹 : 𝑌 –onto→ 𝑋 )  ∧  𝐴  ⊆  𝑋 )  ∧  ( 𝑦  ∈  𝐿  ∧  ( 𝐹  “  𝑦 )  ⊆  𝐴 ) )  →  𝐿  ∈  ( Fil ‘ 𝑌 ) ) | 
						
							| 16 |  | simprl | ⊢ ( ( ( ( 𝑋  ∈  V  ∧  𝐵  ∈  ( fBas ‘ 𝑌 )  ∧  𝐹 : 𝑌 –onto→ 𝑋 )  ∧  𝐴  ⊆  𝑋 )  ∧  ( 𝑦  ∈  𝐿  ∧  ( 𝐹  “  𝑦 )  ⊆  𝐴 ) )  →  𝑦  ∈  𝐿 ) | 
						
							| 17 |  | cnvimass | ⊢ ( ◡ 𝐹  “  𝐴 )  ⊆  dom  𝐹 | 
						
							| 18 |  | fofn | ⊢ ( 𝐹 : 𝑌 –onto→ 𝑋  →  𝐹  Fn  𝑌 ) | 
						
							| 19 | 18 | fndmd | ⊢ ( 𝐹 : 𝑌 –onto→ 𝑋  →  dom  𝐹  =  𝑌 ) | 
						
							| 20 | 17 19 | sseqtrid | ⊢ ( 𝐹 : 𝑌 –onto→ 𝑋  →  ( ◡ 𝐹  “  𝐴 )  ⊆  𝑌 ) | 
						
							| 21 | 20 | 3ad2ant3 | ⊢ ( ( 𝑋  ∈  V  ∧  𝐵  ∈  ( fBas ‘ 𝑌 )  ∧  𝐹 : 𝑌 –onto→ 𝑋 )  →  ( ◡ 𝐹  “  𝐴 )  ⊆  𝑌 ) | 
						
							| 22 | 21 | ad2antrr | ⊢ ( ( ( ( 𝑋  ∈  V  ∧  𝐵  ∈  ( fBas ‘ 𝑌 )  ∧  𝐹 : 𝑌 –onto→ 𝑋 )  ∧  𝐴  ⊆  𝑋 )  ∧  ( 𝑦  ∈  𝐿  ∧  ( 𝐹  “  𝑦 )  ⊆  𝐴 ) )  →  ( ◡ 𝐹  “  𝐴 )  ⊆  𝑌 ) | 
						
							| 23 | 4 | 3ad2ant3 | ⊢ ( ( 𝑋  ∈  V  ∧  𝐵  ∈  ( fBas ‘ 𝑌 )  ∧  𝐹 : 𝑌 –onto→ 𝑋 )  →  Fun  𝐹 ) | 
						
							| 24 | 23 | ad2antrr | ⊢ ( ( ( ( 𝑋  ∈  V  ∧  𝐵  ∈  ( fBas ‘ 𝑌 )  ∧  𝐹 : 𝑌 –onto→ 𝑋 )  ∧  𝐴  ⊆  𝑋 )  ∧  𝑦  ∈  𝐿 )  →  Fun  𝐹 ) | 
						
							| 25 | 1 | eleq2i | ⊢ ( 𝑦  ∈  𝐿  ↔  𝑦  ∈  ( 𝑌 filGen 𝐵 ) ) | 
						
							| 26 |  | elfg | ⊢ ( 𝐵  ∈  ( fBas ‘ 𝑌 )  →  ( 𝑦  ∈  ( 𝑌 filGen 𝐵 )  ↔  ( 𝑦  ⊆  𝑌  ∧  ∃ 𝑧  ∈  𝐵 𝑧  ⊆  𝑦 ) ) ) | 
						
							| 27 | 26 | 3ad2ant2 | ⊢ ( ( 𝑋  ∈  V  ∧  𝐵  ∈  ( fBas ‘ 𝑌 )  ∧  𝐹 : 𝑌 –onto→ 𝑋 )  →  ( 𝑦  ∈  ( 𝑌 filGen 𝐵 )  ↔  ( 𝑦  ⊆  𝑌  ∧  ∃ 𝑧  ∈  𝐵 𝑧  ⊆  𝑦 ) ) ) | 
						
							| 28 | 27 | adantr | ⊢ ( ( ( 𝑋  ∈  V  ∧  𝐵  ∈  ( fBas ‘ 𝑌 )  ∧  𝐹 : 𝑌 –onto→ 𝑋 )  ∧  𝐴  ⊆  𝑋 )  →  ( 𝑦  ∈  ( 𝑌 filGen 𝐵 )  ↔  ( 𝑦  ⊆  𝑌  ∧  ∃ 𝑧  ∈  𝐵 𝑧  ⊆  𝑦 ) ) ) | 
						
							| 29 | 25 28 | bitrid | ⊢ ( ( ( 𝑋  ∈  V  ∧  𝐵  ∈  ( fBas ‘ 𝑌 )  ∧  𝐹 : 𝑌 –onto→ 𝑋 )  ∧  𝐴  ⊆  𝑋 )  →  ( 𝑦  ∈  𝐿  ↔  ( 𝑦  ⊆  𝑌  ∧  ∃ 𝑧  ∈  𝐵 𝑧  ⊆  𝑦 ) ) ) | 
						
							| 30 | 29 | simprbda | ⊢ ( ( ( ( 𝑋  ∈  V  ∧  𝐵  ∈  ( fBas ‘ 𝑌 )  ∧  𝐹 : 𝑌 –onto→ 𝑋 )  ∧  𝐴  ⊆  𝑋 )  ∧  𝑦  ∈  𝐿 )  →  𝑦  ⊆  𝑌 ) | 
						
							| 31 |  | sseq2 | ⊢ ( dom  𝐹  =  𝑌  →  ( 𝑦  ⊆  dom  𝐹  ↔  𝑦  ⊆  𝑌 ) ) | 
						
							| 32 | 31 | biimpar | ⊢ ( ( dom  𝐹  =  𝑌  ∧  𝑦  ⊆  𝑌 )  →  𝑦  ⊆  dom  𝐹 ) | 
						
							| 33 | 19 32 | sylan | ⊢ ( ( 𝐹 : 𝑌 –onto→ 𝑋  ∧  𝑦  ⊆  𝑌 )  →  𝑦  ⊆  dom  𝐹 ) | 
						
							| 34 | 33 | 3ad2antl3 | ⊢ ( ( ( 𝑋  ∈  V  ∧  𝐵  ∈  ( fBas ‘ 𝑌 )  ∧  𝐹 : 𝑌 –onto→ 𝑋 )  ∧  𝑦  ⊆  𝑌 )  →  𝑦  ⊆  dom  𝐹 ) | 
						
							| 35 | 34 | adantlr | ⊢ ( ( ( ( 𝑋  ∈  V  ∧  𝐵  ∈  ( fBas ‘ 𝑌 )  ∧  𝐹 : 𝑌 –onto→ 𝑋 )  ∧  𝐴  ⊆  𝑋 )  ∧  𝑦  ⊆  𝑌 )  →  𝑦  ⊆  dom  𝐹 ) | 
						
							| 36 | 30 35 | syldan | ⊢ ( ( ( ( 𝑋  ∈  V  ∧  𝐵  ∈  ( fBas ‘ 𝑌 )  ∧  𝐹 : 𝑌 –onto→ 𝑋 )  ∧  𝐴  ⊆  𝑋 )  ∧  𝑦  ∈  𝐿 )  →  𝑦  ⊆  dom  𝐹 ) | 
						
							| 37 |  | funimass3 | ⊢ ( ( Fun  𝐹  ∧  𝑦  ⊆  dom  𝐹 )  →  ( ( 𝐹  “  𝑦 )  ⊆  𝐴  ↔  𝑦  ⊆  ( ◡ 𝐹  “  𝐴 ) ) ) | 
						
							| 38 | 24 36 37 | syl2anc | ⊢ ( ( ( ( 𝑋  ∈  V  ∧  𝐵  ∈  ( fBas ‘ 𝑌 )  ∧  𝐹 : 𝑌 –onto→ 𝑋 )  ∧  𝐴  ⊆  𝑋 )  ∧  𝑦  ∈  𝐿 )  →  ( ( 𝐹  “  𝑦 )  ⊆  𝐴  ↔  𝑦  ⊆  ( ◡ 𝐹  “  𝐴 ) ) ) | 
						
							| 39 | 38 | biimpd | ⊢ ( ( ( ( 𝑋  ∈  V  ∧  𝐵  ∈  ( fBas ‘ 𝑌 )  ∧  𝐹 : 𝑌 –onto→ 𝑋 )  ∧  𝐴  ⊆  𝑋 )  ∧  𝑦  ∈  𝐿 )  →  ( ( 𝐹  “  𝑦 )  ⊆  𝐴  →  𝑦  ⊆  ( ◡ 𝐹  “  𝐴 ) ) ) | 
						
							| 40 | 39 | impr | ⊢ ( ( ( ( 𝑋  ∈  V  ∧  𝐵  ∈  ( fBas ‘ 𝑌 )  ∧  𝐹 : 𝑌 –onto→ 𝑋 )  ∧  𝐴  ⊆  𝑋 )  ∧  ( 𝑦  ∈  𝐿  ∧  ( 𝐹  “  𝑦 )  ⊆  𝐴 ) )  →  𝑦  ⊆  ( ◡ 𝐹  “  𝐴 ) ) | 
						
							| 41 |  | filss | ⊢ ( ( 𝐿  ∈  ( Fil ‘ 𝑌 )  ∧  ( 𝑦  ∈  𝐿  ∧  ( ◡ 𝐹  “  𝐴 )  ⊆  𝑌  ∧  𝑦  ⊆  ( ◡ 𝐹  “  𝐴 ) ) )  →  ( ◡ 𝐹  “  𝐴 )  ∈  𝐿 ) | 
						
							| 42 | 15 16 22 40 41 | syl13anc | ⊢ ( ( ( ( 𝑋  ∈  V  ∧  𝐵  ∈  ( fBas ‘ 𝑌 )  ∧  𝐹 : 𝑌 –onto→ 𝑋 )  ∧  𝐴  ⊆  𝑋 )  ∧  ( 𝑦  ∈  𝐿  ∧  ( 𝐹  “  𝑦 )  ⊆  𝐴 ) )  →  ( ◡ 𝐹  “  𝐴 )  ∈  𝐿 ) | 
						
							| 43 |  | foimacnv | ⊢ ( ( 𝐹 : 𝑌 –onto→ 𝑋  ∧  𝐴  ⊆  𝑋 )  →  ( 𝐹  “  ( ◡ 𝐹  “  𝐴 ) )  =  𝐴 ) | 
						
							| 44 | 43 | eqcomd | ⊢ ( ( 𝐹 : 𝑌 –onto→ 𝑋  ∧  𝐴  ⊆  𝑋 )  →  𝐴  =  ( 𝐹  “  ( ◡ 𝐹  “  𝐴 ) ) ) | 
						
							| 45 | 44 | 3ad2antl3 | ⊢ ( ( ( 𝑋  ∈  V  ∧  𝐵  ∈  ( fBas ‘ 𝑌 )  ∧  𝐹 : 𝑌 –onto→ 𝑋 )  ∧  𝐴  ⊆  𝑋 )  →  𝐴  =  ( 𝐹  “  ( ◡ 𝐹  “  𝐴 ) ) ) | 
						
							| 46 | 45 | adantr | ⊢ ( ( ( ( 𝑋  ∈  V  ∧  𝐵  ∈  ( fBas ‘ 𝑌 )  ∧  𝐹 : 𝑌 –onto→ 𝑋 )  ∧  𝐴  ⊆  𝑋 )  ∧  ( 𝑦  ∈  𝐿  ∧  ( 𝐹  “  𝑦 )  ⊆  𝐴 ) )  →  𝐴  =  ( 𝐹  “  ( ◡ 𝐹  “  𝐴 ) ) ) | 
						
							| 47 |  | imaeq2 | ⊢ ( 𝑥  =  ( ◡ 𝐹  “  𝐴 )  →  ( 𝐹  “  𝑥 )  =  ( 𝐹  “  ( ◡ 𝐹  “  𝐴 ) ) ) | 
						
							| 48 | 47 | rspceeqv | ⊢ ( ( ( ◡ 𝐹  “  𝐴 )  ∈  𝐿  ∧  𝐴  =  ( 𝐹  “  ( ◡ 𝐹  “  𝐴 ) ) )  →  ∃ 𝑥  ∈  𝐿 𝐴  =  ( 𝐹  “  𝑥 ) ) | 
						
							| 49 | 42 46 48 | syl2anc | ⊢ ( ( ( ( 𝑋  ∈  V  ∧  𝐵  ∈  ( fBas ‘ 𝑌 )  ∧  𝐹 : 𝑌 –onto→ 𝑋 )  ∧  𝐴  ⊆  𝑋 )  ∧  ( 𝑦  ∈  𝐿  ∧  ( 𝐹  “  𝑦 )  ⊆  𝐴 ) )  →  ∃ 𝑥  ∈  𝐿 𝐴  =  ( 𝐹  “  𝑥 ) ) | 
						
							| 50 | 49 | rexlimdvaa | ⊢ ( ( ( 𝑋  ∈  V  ∧  𝐵  ∈  ( fBas ‘ 𝑌 )  ∧  𝐹 : 𝑌 –onto→ 𝑋 )  ∧  𝐴  ⊆  𝑋 )  →  ( ∃ 𝑦  ∈  𝐿 ( 𝐹  “  𝑦 )  ⊆  𝐴  →  ∃ 𝑥  ∈  𝐿 𝐴  =  ( 𝐹  “  𝑥 ) ) ) | 
						
							| 51 | 50 | expimpd | ⊢ ( ( 𝑋  ∈  V  ∧  𝐵  ∈  ( fBas ‘ 𝑌 )  ∧  𝐹 : 𝑌 –onto→ 𝑋 )  →  ( ( 𝐴  ⊆  𝑋  ∧  ∃ 𝑦  ∈  𝐿 ( 𝐹  “  𝑦 )  ⊆  𝐴 )  →  ∃ 𝑥  ∈  𝐿 𝐴  =  ( 𝐹  “  𝑥 ) ) ) | 
						
							| 52 |  | simprr | ⊢ ( ( ( 𝑋  ∈  V  ∧  𝐵  ∈  ( fBas ‘ 𝑌 )  ∧  𝐹 : 𝑌 –onto→ 𝑋 )  ∧  ( 𝑥  ∈  𝐿  ∧  𝐴  =  ( 𝐹  “  𝑥 ) ) )  →  𝐴  =  ( 𝐹  “  𝑥 ) ) | 
						
							| 53 |  | imassrn | ⊢ ( 𝐹  “  𝑥 )  ⊆  ran  𝐹 | 
						
							| 54 |  | forn | ⊢ ( 𝐹 : 𝑌 –onto→ 𝑋  →  ran  𝐹  =  𝑋 ) | 
						
							| 55 | 54 | 3ad2ant3 | ⊢ ( ( 𝑋  ∈  V  ∧  𝐵  ∈  ( fBas ‘ 𝑌 )  ∧  𝐹 : 𝑌 –onto→ 𝑋 )  →  ran  𝐹  =  𝑋 ) | 
						
							| 56 | 55 | adantr | ⊢ ( ( ( 𝑋  ∈  V  ∧  𝐵  ∈  ( fBas ‘ 𝑌 )  ∧  𝐹 : 𝑌 –onto→ 𝑋 )  ∧  ( 𝑥  ∈  𝐿  ∧  𝐴  =  ( 𝐹  “  𝑥 ) ) )  →  ran  𝐹  =  𝑋 ) | 
						
							| 57 | 53 56 | sseqtrid | ⊢ ( ( ( 𝑋  ∈  V  ∧  𝐵  ∈  ( fBas ‘ 𝑌 )  ∧  𝐹 : 𝑌 –onto→ 𝑋 )  ∧  ( 𝑥  ∈  𝐿  ∧  𝐴  =  ( 𝐹  “  𝑥 ) ) )  →  ( 𝐹  “  𝑥 )  ⊆  𝑋 ) | 
						
							| 58 | 52 57 | eqsstrd | ⊢ ( ( ( 𝑋  ∈  V  ∧  𝐵  ∈  ( fBas ‘ 𝑌 )  ∧  𝐹 : 𝑌 –onto→ 𝑋 )  ∧  ( 𝑥  ∈  𝐿  ∧  𝐴  =  ( 𝐹  “  𝑥 ) ) )  →  𝐴  ⊆  𝑋 ) | 
						
							| 59 |  | eqimss2 | ⊢ ( 𝐴  =  ( 𝐹  “  𝑥 )  →  ( 𝐹  “  𝑥 )  ⊆  𝐴 ) | 
						
							| 60 |  | imaeq2 | ⊢ ( 𝑦  =  𝑥  →  ( 𝐹  “  𝑦 )  =  ( 𝐹  “  𝑥 ) ) | 
						
							| 61 | 60 | sseq1d | ⊢ ( 𝑦  =  𝑥  →  ( ( 𝐹  “  𝑦 )  ⊆  𝐴  ↔  ( 𝐹  “  𝑥 )  ⊆  𝐴 ) ) | 
						
							| 62 | 61 | rspcev | ⊢ ( ( 𝑥  ∈  𝐿  ∧  ( 𝐹  “  𝑥 )  ⊆  𝐴 )  →  ∃ 𝑦  ∈  𝐿 ( 𝐹  “  𝑦 )  ⊆  𝐴 ) | 
						
							| 63 | 59 62 | sylan2 | ⊢ ( ( 𝑥  ∈  𝐿  ∧  𝐴  =  ( 𝐹  “  𝑥 ) )  →  ∃ 𝑦  ∈  𝐿 ( 𝐹  “  𝑦 )  ⊆  𝐴 ) | 
						
							| 64 | 63 | adantl | ⊢ ( ( ( 𝑋  ∈  V  ∧  𝐵  ∈  ( fBas ‘ 𝑌 )  ∧  𝐹 : 𝑌 –onto→ 𝑋 )  ∧  ( 𝑥  ∈  𝐿  ∧  𝐴  =  ( 𝐹  “  𝑥 ) ) )  →  ∃ 𝑦  ∈  𝐿 ( 𝐹  “  𝑦 )  ⊆  𝐴 ) | 
						
							| 65 | 58 64 | jca | ⊢ ( ( ( 𝑋  ∈  V  ∧  𝐵  ∈  ( fBas ‘ 𝑌 )  ∧  𝐹 : 𝑌 –onto→ 𝑋 )  ∧  ( 𝑥  ∈  𝐿  ∧  𝐴  =  ( 𝐹  “  𝑥 ) ) )  →  ( 𝐴  ⊆  𝑋  ∧  ∃ 𝑦  ∈  𝐿 ( 𝐹  “  𝑦 )  ⊆  𝐴 ) ) | 
						
							| 66 | 65 | rexlimdvaa | ⊢ ( ( 𝑋  ∈  V  ∧  𝐵  ∈  ( fBas ‘ 𝑌 )  ∧  𝐹 : 𝑌 –onto→ 𝑋 )  →  ( ∃ 𝑥  ∈  𝐿 𝐴  =  ( 𝐹  “  𝑥 )  →  ( 𝐴  ⊆  𝑋  ∧  ∃ 𝑦  ∈  𝐿 ( 𝐹  “  𝑦 )  ⊆  𝐴 ) ) ) | 
						
							| 67 | 51 66 | impbid | ⊢ ( ( 𝑋  ∈  V  ∧  𝐵  ∈  ( fBas ‘ 𝑌 )  ∧  𝐹 : 𝑌 –onto→ 𝑋 )  →  ( ( 𝐴  ⊆  𝑋  ∧  ∃ 𝑦  ∈  𝐿 ( 𝐹  “  𝑦 )  ⊆  𝐴 )  ↔  ∃ 𝑥  ∈  𝐿 𝐴  =  ( 𝐹  “  𝑥 ) ) ) | 
						
							| 68 | 11 67 | bitrd | ⊢ ( ( 𝑋  ∈  V  ∧  𝐵  ∈  ( fBas ‘ 𝑌 )  ∧  𝐹 : 𝑌 –onto→ 𝑋 )  →  ( 𝐴  ∈  ( ( 𝑋  FilMap  𝐹 ) ‘ 𝐵 )  ↔  ∃ 𝑥  ∈  𝐿 𝐴  =  ( 𝐹  “  𝑥 ) ) ) | 
						
							| 69 | 68 | 3coml | ⊢ ( ( 𝐵  ∈  ( fBas ‘ 𝑌 )  ∧  𝐹 : 𝑌 –onto→ 𝑋  ∧  𝑋  ∈  V )  →  ( 𝐴  ∈  ( ( 𝑋  FilMap  𝐹 ) ‘ 𝐵 )  ↔  ∃ 𝑥  ∈  𝐿 𝐴  =  ( 𝐹  “  𝑥 ) ) ) | 
						
							| 70 | 8 69 | mpd3an3 | ⊢ ( ( 𝐵  ∈  ( fBas ‘ 𝑌 )  ∧  𝐹 : 𝑌 –onto→ 𝑋 )  →  ( 𝐴  ∈  ( ( 𝑋  FilMap  𝐹 ) ‘ 𝐵 )  ↔  ∃ 𝑥  ∈  𝐿 𝐴  =  ( 𝐹  “  𝑥 ) ) ) |