What's this do? Is it something like:

```
foo bar baz qux
        ^~~~~~~ == span
```
then `sub_span` is like "Span { lo: 4, hi: 7, ... }", leaving the return value as

```
foo bar baz qux
            ^~~
```

If it is that, why is it going through `lookup_byte_offset` (since it presumably wouldn't correspond to a true file position).

If it's not that, why is it adding byte positions?