Description: Extract the third symbol from a length 4 string. (Contributed by Thierry Arnoux, 8-Oct-2020)
Ref | Expression | ||
---|---|---|---|
Assertion | s4fv2 | |
Step | Hyp | Ref | Expression |
---|---|---|---|
1 | df-s4 | |
|
2 | s3cli | |
|
3 | s3len | |
|
4 | s3fv2 | |
|
5 | 2nn0 | |
|
6 | 2lt3 | |
|
7 | 1 2 3 4 5 6 | cats1fv | |