regex_constants::match_partial docs indecipherable

[the docs](https://www.boost.org/doc/libs/1_79_0_beta1/doc/html/boost/xpressive/regex_constants/match_flag_type.html)
say:
```
match_partial

Specifies that if no match can be found, then it is acceptable to return a match [from, last) where from != last, if there exists some sequence of characters [from,to) of which [from,last) is a prefix, and which would result in a full match.
```
with no explanation of what `from`, `last` and `to` are. One would guess that [from,last] is the string to be searched; however,
if that's so, then there cannot be a [from,to] where [from,last] is a prefix, because that would require `last < to`, which can't
be since the search string cannot extend past `last`.

Please clarify.