| Step | Hyp | Ref | Expression | 
						
							| 1 |  | swrdccatin2.l |  |-  L = ( # ` A ) | 
						
							| 2 |  | ccatcl |  |-  ( ( A e. Word V /\ B e. Word V ) -> ( A ++ B ) e. Word V ) | 
						
							| 3 | 2 | adantr |  |-  ( ( ( A e. Word V /\ B e. Word V ) /\ ( M e. ( 0 ... L ) /\ N e. ( L ... ( L + ( # ` B ) ) ) ) ) -> ( A ++ B ) e. Word V ) | 
						
							| 4 |  | elfz0fzfz0 |  |-  ( ( M e. ( 0 ... L ) /\ N e. ( L ... ( L + ( # ` B ) ) ) ) -> M e. ( 0 ... N ) ) | 
						
							| 5 | 4 | adantl |  |-  ( ( ( A e. Word V /\ B e. Word V ) /\ ( M e. ( 0 ... L ) /\ N e. ( L ... ( L + ( # ` B ) ) ) ) ) -> M e. ( 0 ... N ) ) | 
						
							| 6 |  | elfzuz2 |  |-  ( M e. ( 0 ... L ) -> L e. ( ZZ>= ` 0 ) ) | 
						
							| 7 |  | fzss1 |  |-  ( L e. ( ZZ>= ` 0 ) -> ( L ... ( L + ( # ` B ) ) ) C_ ( 0 ... ( L + ( # ` B ) ) ) ) | 
						
							| 8 | 6 7 | syl |  |-  ( M e. ( 0 ... L ) -> ( L ... ( L + ( # ` B ) ) ) C_ ( 0 ... ( L + ( # ` B ) ) ) ) | 
						
							| 9 | 8 | sselda |  |-  ( ( M e. ( 0 ... L ) /\ N e. ( L ... ( L + ( # ` B ) ) ) ) -> N e. ( 0 ... ( L + ( # ` B ) ) ) ) | 
						
							| 10 |  | ccatlen |  |-  ( ( A e. Word V /\ B e. Word V ) -> ( # ` ( A ++ B ) ) = ( ( # ` A ) + ( # ` B ) ) ) | 
						
							| 11 | 1 | oveq1i |  |-  ( L + ( # ` B ) ) = ( ( # ` A ) + ( # ` B ) ) | 
						
							| 12 | 10 11 | eqtr4di |  |-  ( ( A e. Word V /\ B e. Word V ) -> ( # ` ( A ++ B ) ) = ( L + ( # ` B ) ) ) | 
						
							| 13 | 12 | oveq2d |  |-  ( ( A e. Word V /\ B e. Word V ) -> ( 0 ... ( # ` ( A ++ B ) ) ) = ( 0 ... ( L + ( # ` B ) ) ) ) | 
						
							| 14 | 13 | eleq2d |  |-  ( ( A e. Word V /\ B e. Word V ) -> ( N e. ( 0 ... ( # ` ( A ++ B ) ) ) <-> N e. ( 0 ... ( L + ( # ` B ) ) ) ) ) | 
						
							| 15 | 9 14 | imbitrrid |  |-  ( ( A e. Word V /\ B e. Word V ) -> ( ( M e. ( 0 ... L ) /\ N e. ( L ... ( L + ( # ` B ) ) ) ) -> N e. ( 0 ... ( # ` ( A ++ B ) ) ) ) ) | 
						
							| 16 | 15 | imp |  |-  ( ( ( A e. Word V /\ B e. Word V ) /\ ( M e. ( 0 ... L ) /\ N e. ( L ... ( L + ( # ` B ) ) ) ) ) -> N e. ( 0 ... ( # ` ( A ++ B ) ) ) ) | 
						
							| 17 | 3 5 16 | 3jca |  |-  ( ( ( A e. Word V /\ B e. Word V ) /\ ( M e. ( 0 ... L ) /\ N e. ( L ... ( L + ( # ` B ) ) ) ) ) -> ( ( A ++ B ) e. Word V /\ M e. ( 0 ... N ) /\ N e. ( 0 ... ( # ` ( A ++ B ) ) ) ) ) |