A Rust version for fractional-indexing .
Rewritten from existed codebase:
Docs.
```rs use lexiconfractionalindex::generatekeybetween;
generatekeybetween("a0", "a0V")
generatekeybetween("a0", "") // generate key between "a0" and ending
generatekeybetween("", "a0") // generate key between beggining and "a0"
generatekeybetween("", "") // generate first key, no sisters ```
TODO