| Step | Hyp | Ref | Expression | 
						
							| 1 |  | str.1 |  | 
						
							| 2 |  | str.2 |  | 
						
							| 3 |  | dfral2 |  | 
						
							| 4 | 1 2 | strlem1 |  | 
						
							| 5 |  | eqid |  | 
						
							| 6 |  | biid |  | 
						
							| 7 | 5 6 1 2 | strlem3 |  | 
						
							| 8 | 5 6 1 2 | strlem6 |  | 
						
							| 9 |  | fveq1 |  | 
						
							| 10 | 9 | eqeq1d |  | 
						
							| 11 |  | fveq1 |  | 
						
							| 12 | 11 | eqeq1d |  | 
						
							| 13 | 10 12 | imbi12d |  | 
						
							| 14 | 13 | notbid |  | 
						
							| 15 | 14 | rspcev |  | 
						
							| 16 | 7 8 15 | syl2anc |  | 
						
							| 17 | 16 | rexlimiva |  | 
						
							| 18 | 4 17 | syl |  | 
						
							| 19 | 18 | con1i |  | 
						
							| 20 | 3 19 | sylbi |  |