| Step | Hyp | Ref | Expression | 
						
							| 1 |  | mdsym.1 |  |-  A e. CH | 
						
							| 2 |  | mdsym.2 |  |-  B e. CH | 
						
							| 3 | 2 | choccli |  |-  ( _|_ ` B ) e. CH | 
						
							| 4 | 1 | choccli |  |-  ( _|_ ` A ) e. CH | 
						
							| 5 |  | eqid |  |-  ( ( _|_ ` B ) vH x ) = ( ( _|_ ` B ) vH x ) | 
						
							| 6 | 3 4 5 | mdsymlem8 |  |-  ( ( ( _|_ ` B ) =/= 0H /\ ( _|_ ` A ) =/= 0H ) -> ( ( _|_ ` A ) MH* ( _|_ ` B ) <-> ( _|_ ` B ) MH* ( _|_ ` A ) ) ) | 
						
							| 7 |  | mddmd |  |-  ( ( A e. CH /\ B e. CH ) -> ( A MH B <-> ( _|_ ` A ) MH* ( _|_ ` B ) ) ) | 
						
							| 8 | 1 2 7 | mp2an |  |-  ( A MH B <-> ( _|_ ` A ) MH* ( _|_ ` B ) ) | 
						
							| 9 |  | mddmd |  |-  ( ( B e. CH /\ A e. CH ) -> ( B MH A <-> ( _|_ ` B ) MH* ( _|_ ` A ) ) ) | 
						
							| 10 | 2 1 9 | mp2an |  |-  ( B MH A <-> ( _|_ ` B ) MH* ( _|_ ` A ) ) | 
						
							| 11 | 6 8 10 | 3bitr4g |  |-  ( ( ( _|_ ` B ) =/= 0H /\ ( _|_ ` A ) =/= 0H ) -> ( A MH B <-> B MH A ) ) | 
						
							| 12 | 1 | chssii |  |-  A C_ ~H | 
						
							| 13 |  | fveq2 |  |-  ( ( _|_ ` B ) = 0H -> ( _|_ ` ( _|_ ` B ) ) = ( _|_ ` 0H ) ) | 
						
							| 14 | 2 | pjococi |  |-  ( _|_ ` ( _|_ ` B ) ) = B | 
						
							| 15 |  | choc0 |  |-  ( _|_ ` 0H ) = ~H | 
						
							| 16 | 13 14 15 | 3eqtr3g |  |-  ( ( _|_ ` B ) = 0H -> B = ~H ) | 
						
							| 17 | 12 16 | sseqtrrid |  |-  ( ( _|_ ` B ) = 0H -> A C_ B ) | 
						
							| 18 |  | ssmd1 |  |-  ( ( A e. CH /\ B e. CH /\ A C_ B ) -> A MH B ) | 
						
							| 19 | 1 2 18 | mp3an12 |  |-  ( A C_ B -> A MH B ) | 
						
							| 20 |  | ssmd2 |  |-  ( ( A e. CH /\ B e. CH /\ A C_ B ) -> B MH A ) | 
						
							| 21 | 1 2 20 | mp3an12 |  |-  ( A C_ B -> B MH A ) | 
						
							| 22 | 19 21 | jca |  |-  ( A C_ B -> ( A MH B /\ B MH A ) ) | 
						
							| 23 |  | pm5.1 |  |-  ( ( A MH B /\ B MH A ) -> ( A MH B <-> B MH A ) ) | 
						
							| 24 | 17 22 23 | 3syl |  |-  ( ( _|_ ` B ) = 0H -> ( A MH B <-> B MH A ) ) | 
						
							| 25 | 2 | chssii |  |-  B C_ ~H | 
						
							| 26 |  | fveq2 |  |-  ( ( _|_ ` A ) = 0H -> ( _|_ ` ( _|_ ` A ) ) = ( _|_ ` 0H ) ) | 
						
							| 27 | 1 | pjococi |  |-  ( _|_ ` ( _|_ ` A ) ) = A | 
						
							| 28 | 26 27 15 | 3eqtr3g |  |-  ( ( _|_ ` A ) = 0H -> A = ~H ) | 
						
							| 29 | 25 28 | sseqtrrid |  |-  ( ( _|_ ` A ) = 0H -> B C_ A ) | 
						
							| 30 |  | ssmd2 |  |-  ( ( B e. CH /\ A e. CH /\ B C_ A ) -> A MH B ) | 
						
							| 31 | 2 1 30 | mp3an12 |  |-  ( B C_ A -> A MH B ) | 
						
							| 32 |  | ssmd1 |  |-  ( ( B e. CH /\ A e. CH /\ B C_ A ) -> B MH A ) | 
						
							| 33 | 2 1 32 | mp3an12 |  |-  ( B C_ A -> B MH A ) | 
						
							| 34 | 31 33 | jca |  |-  ( B C_ A -> ( A MH B /\ B MH A ) ) | 
						
							| 35 | 29 34 23 | 3syl |  |-  ( ( _|_ ` A ) = 0H -> ( A MH B <-> B MH A ) ) | 
						
							| 36 | 11 24 35 | pm2.61iine |  |-  ( A MH B <-> B MH A ) |