| Step | Hyp | Ref | Expression | 
						
							| 1 |  | eceqoveq.5 |  |-  .~ Er ( S X. S ) | 
						
							| 2 |  | eceqoveq.7 |  |-  dom .+ = ( S X. S ) | 
						
							| 3 |  | eceqoveq.8 |  |-  -. (/) e. S | 
						
							| 4 |  | eceqoveq.9 |  |-  ( ( x e. S /\ y e. S ) -> ( x .+ y ) e. S ) | 
						
							| 5 |  | eceqoveq.10 |  |-  ( ( ( A e. S /\ B e. S ) /\ ( C e. S /\ D e. S ) ) -> ( <. A , B >. .~ <. C , D >. <-> ( A .+ D ) = ( B .+ C ) ) ) | 
						
							| 6 |  | opelxpi |  |-  ( ( A e. S /\ B e. S ) -> <. A , B >. e. ( S X. S ) ) | 
						
							| 7 | 6 | ad2antrr |  |-  ( ( ( ( A e. S /\ B e. S ) /\ C e. S ) /\ [ <. A , B >. ] .~ = [ <. C , D >. ] .~ ) -> <. A , B >. e. ( S X. S ) ) | 
						
							| 8 | 1 | a1i |  |-  ( ( ( ( A e. S /\ B e. S ) /\ C e. S ) /\ [ <. A , B >. ] .~ = [ <. C , D >. ] .~ ) -> .~ Er ( S X. S ) ) | 
						
							| 9 |  | simpr |  |-  ( ( ( ( A e. S /\ B e. S ) /\ C e. S ) /\ [ <. A , B >. ] .~ = [ <. C , D >. ] .~ ) -> [ <. A , B >. ] .~ = [ <. C , D >. ] .~ ) | 
						
							| 10 | 8 9 | ereldm |  |-  ( ( ( ( A e. S /\ B e. S ) /\ C e. S ) /\ [ <. A , B >. ] .~ = [ <. C , D >. ] .~ ) -> ( <. A , B >. e. ( S X. S ) <-> <. C , D >. e. ( S X. S ) ) ) | 
						
							| 11 | 7 10 | mpbid |  |-  ( ( ( ( A e. S /\ B e. S ) /\ C e. S ) /\ [ <. A , B >. ] .~ = [ <. C , D >. ] .~ ) -> <. C , D >. e. ( S X. S ) ) | 
						
							| 12 |  | opelxp2 |  |-  ( <. C , D >. e. ( S X. S ) -> D e. S ) | 
						
							| 13 | 11 12 | syl |  |-  ( ( ( ( A e. S /\ B e. S ) /\ C e. S ) /\ [ <. A , B >. ] .~ = [ <. C , D >. ] .~ ) -> D e. S ) | 
						
							| 14 | 13 | ex |  |-  ( ( ( A e. S /\ B e. S ) /\ C e. S ) -> ( [ <. A , B >. ] .~ = [ <. C , D >. ] .~ -> D e. S ) ) | 
						
							| 15 | 4 | caovcl |  |-  ( ( B e. S /\ C e. S ) -> ( B .+ C ) e. S ) | 
						
							| 16 |  | eleq1 |  |-  ( ( A .+ D ) = ( B .+ C ) -> ( ( A .+ D ) e. S <-> ( B .+ C ) e. S ) ) | 
						
							| 17 | 15 16 | imbitrrid |  |-  ( ( A .+ D ) = ( B .+ C ) -> ( ( B e. S /\ C e. S ) -> ( A .+ D ) e. S ) ) | 
						
							| 18 | 2 3 | ndmovrcl |  |-  ( ( A .+ D ) e. S -> ( A e. S /\ D e. S ) ) | 
						
							| 19 | 18 | simprd |  |-  ( ( A .+ D ) e. S -> D e. S ) | 
						
							| 20 | 17 19 | syl6com |  |-  ( ( B e. S /\ C e. S ) -> ( ( A .+ D ) = ( B .+ C ) -> D e. S ) ) | 
						
							| 21 | 20 | adantll |  |-  ( ( ( A e. S /\ B e. S ) /\ C e. S ) -> ( ( A .+ D ) = ( B .+ C ) -> D e. S ) ) | 
						
							| 22 | 1 | a1i |  |-  ( ( ( A e. S /\ B e. S ) /\ ( C e. S /\ D e. S ) ) -> .~ Er ( S X. S ) ) | 
						
							| 23 | 6 | adantr |  |-  ( ( ( A e. S /\ B e. S ) /\ ( C e. S /\ D e. S ) ) -> <. A , B >. e. ( S X. S ) ) | 
						
							| 24 | 22 23 | erth |  |-  ( ( ( A e. S /\ B e. S ) /\ ( C e. S /\ D e. S ) ) -> ( <. A , B >. .~ <. C , D >. <-> [ <. A , B >. ] .~ = [ <. C , D >. ] .~ ) ) | 
						
							| 25 | 24 5 | bitr3d |  |-  ( ( ( A e. S /\ B e. S ) /\ ( C e. S /\ D e. S ) ) -> ( [ <. A , B >. ] .~ = [ <. C , D >. ] .~ <-> ( A .+ D ) = ( B .+ C ) ) ) | 
						
							| 26 | 25 | expr |  |-  ( ( ( A e. S /\ B e. S ) /\ C e. S ) -> ( D e. S -> ( [ <. A , B >. ] .~ = [ <. C , D >. ] .~ <-> ( A .+ D ) = ( B .+ C ) ) ) ) | 
						
							| 27 | 14 21 26 | pm5.21ndd |  |-  ( ( ( A e. S /\ B e. S ) /\ C e. S ) -> ( [ <. A , B >. ] .~ = [ <. C , D >. ] .~ <-> ( A .+ D ) = ( B .+ C ) ) ) | 
						
							| 28 | 27 | an32s |  |-  ( ( ( A e. S /\ C e. S ) /\ B e. S ) -> ( [ <. A , B >. ] .~ = [ <. C , D >. ] .~ <-> ( A .+ D ) = ( B .+ C ) ) ) | 
						
							| 29 |  | eqcom |  |-  ( (/) = [ <. C , D >. ] .~ <-> [ <. C , D >. ] .~ = (/) ) | 
						
							| 30 |  | erdm |  |-  ( .~ Er ( S X. S ) -> dom .~ = ( S X. S ) ) | 
						
							| 31 | 1 30 | ax-mp |  |-  dom .~ = ( S X. S ) | 
						
							| 32 | 31 | eleq2i |  |-  ( <. C , D >. e. dom .~ <-> <. C , D >. e. ( S X. S ) ) | 
						
							| 33 |  | ecdmn0 |  |-  ( <. C , D >. e. dom .~ <-> [ <. C , D >. ] .~ =/= (/) ) | 
						
							| 34 |  | opelxp |  |-  ( <. C , D >. e. ( S X. S ) <-> ( C e. S /\ D e. S ) ) | 
						
							| 35 | 32 33 34 | 3bitr3i |  |-  ( [ <. C , D >. ] .~ =/= (/) <-> ( C e. S /\ D e. S ) ) | 
						
							| 36 | 35 | simplbi2 |  |-  ( C e. S -> ( D e. S -> [ <. C , D >. ] .~ =/= (/) ) ) | 
						
							| 37 | 36 | ad2antlr |  |-  ( ( ( A e. S /\ C e. S ) /\ -. B e. S ) -> ( D e. S -> [ <. C , D >. ] .~ =/= (/) ) ) | 
						
							| 38 | 37 | necon2bd |  |-  ( ( ( A e. S /\ C e. S ) /\ -. B e. S ) -> ( [ <. C , D >. ] .~ = (/) -> -. D e. S ) ) | 
						
							| 39 |  | simpr |  |-  ( ( A e. S /\ D e. S ) -> D e. S ) | 
						
							| 40 | 2 | ndmov |  |-  ( -. ( A e. S /\ D e. S ) -> ( A .+ D ) = (/) ) | 
						
							| 41 | 39 40 | nsyl5 |  |-  ( -. D e. S -> ( A .+ D ) = (/) ) | 
						
							| 42 | 38 41 | syl6 |  |-  ( ( ( A e. S /\ C e. S ) /\ -. B e. S ) -> ( [ <. C , D >. ] .~ = (/) -> ( A .+ D ) = (/) ) ) | 
						
							| 43 |  | eleq1 |  |-  ( ( A .+ D ) = (/) -> ( ( A .+ D ) e. S <-> (/) e. S ) ) | 
						
							| 44 | 3 43 | mtbiri |  |-  ( ( A .+ D ) = (/) -> -. ( A .+ D ) e. S ) | 
						
							| 45 | 35 | simprbi |  |-  ( [ <. C , D >. ] .~ =/= (/) -> D e. S ) | 
						
							| 46 | 4 | caovcl |  |-  ( ( A e. S /\ D e. S ) -> ( A .+ D ) e. S ) | 
						
							| 47 | 46 | ex |  |-  ( A e. S -> ( D e. S -> ( A .+ D ) e. S ) ) | 
						
							| 48 | 47 | ad2antrr |  |-  ( ( ( A e. S /\ C e. S ) /\ -. B e. S ) -> ( D e. S -> ( A .+ D ) e. S ) ) | 
						
							| 49 | 45 48 | syl5 |  |-  ( ( ( A e. S /\ C e. S ) /\ -. B e. S ) -> ( [ <. C , D >. ] .~ =/= (/) -> ( A .+ D ) e. S ) ) | 
						
							| 50 | 49 | necon1bd |  |-  ( ( ( A e. S /\ C e. S ) /\ -. B e. S ) -> ( -. ( A .+ D ) e. S -> [ <. C , D >. ] .~ = (/) ) ) | 
						
							| 51 | 44 50 | syl5 |  |-  ( ( ( A e. S /\ C e. S ) /\ -. B e. S ) -> ( ( A .+ D ) = (/) -> [ <. C , D >. ] .~ = (/) ) ) | 
						
							| 52 | 42 51 | impbid |  |-  ( ( ( A e. S /\ C e. S ) /\ -. B e. S ) -> ( [ <. C , D >. ] .~ = (/) <-> ( A .+ D ) = (/) ) ) | 
						
							| 53 | 29 52 | bitrid |  |-  ( ( ( A e. S /\ C e. S ) /\ -. B e. S ) -> ( (/) = [ <. C , D >. ] .~ <-> ( A .+ D ) = (/) ) ) | 
						
							| 54 | 31 | eleq2i |  |-  ( <. A , B >. e. dom .~ <-> <. A , B >. e. ( S X. S ) ) | 
						
							| 55 |  | ecdmn0 |  |-  ( <. A , B >. e. dom .~ <-> [ <. A , B >. ] .~ =/= (/) ) | 
						
							| 56 |  | opelxp |  |-  ( <. A , B >. e. ( S X. S ) <-> ( A e. S /\ B e. S ) ) | 
						
							| 57 | 54 55 56 | 3bitr3i |  |-  ( [ <. A , B >. ] .~ =/= (/) <-> ( A e. S /\ B e. S ) ) | 
						
							| 58 | 57 | simprbi |  |-  ( [ <. A , B >. ] .~ =/= (/) -> B e. S ) | 
						
							| 59 | 58 | necon1bi |  |-  ( -. B e. S -> [ <. A , B >. ] .~ = (/) ) | 
						
							| 60 | 59 | adantl |  |-  ( ( ( A e. S /\ C e. S ) /\ -. B e. S ) -> [ <. A , B >. ] .~ = (/) ) | 
						
							| 61 | 60 | eqeq1d |  |-  ( ( ( A e. S /\ C e. S ) /\ -. B e. S ) -> ( [ <. A , B >. ] .~ = [ <. C , D >. ] .~ <-> (/) = [ <. C , D >. ] .~ ) ) | 
						
							| 62 |  | simpl |  |-  ( ( B e. S /\ C e. S ) -> B e. S ) | 
						
							| 63 | 2 | ndmov |  |-  ( -. ( B e. S /\ C e. S ) -> ( B .+ C ) = (/) ) | 
						
							| 64 | 62 63 | nsyl5 |  |-  ( -. B e. S -> ( B .+ C ) = (/) ) | 
						
							| 65 | 64 | adantl |  |-  ( ( ( A e. S /\ C e. S ) /\ -. B e. S ) -> ( B .+ C ) = (/) ) | 
						
							| 66 | 65 | eqeq2d |  |-  ( ( ( A e. S /\ C e. S ) /\ -. B e. S ) -> ( ( A .+ D ) = ( B .+ C ) <-> ( A .+ D ) = (/) ) ) | 
						
							| 67 | 53 61 66 | 3bitr4d |  |-  ( ( ( A e. S /\ C e. S ) /\ -. B e. S ) -> ( [ <. A , B >. ] .~ = [ <. C , D >. ] .~ <-> ( A .+ D ) = ( B .+ C ) ) ) | 
						
							| 68 | 28 67 | pm2.61dan |  |-  ( ( A e. S /\ C e. S ) -> ( [ <. A , B >. ] .~ = [ <. C , D >. ] .~ <-> ( A .+ D ) = ( B .+ C ) ) ) |