According to the documentation:

If first is specified, it refers to the first char index in the string to start modifying

$ tclsh8.6
% string tolower ABC 1
AbC

I'm not 100% sure, but I'm leaning towards changing the documentation.

Documentation updated in all branches

> Even not in our test suite? That would be worse.

In our test suite, yes. But not anywhere else (not even in Tk)

> I couldn't find any code which uses "string tolower/toupper/totitle"

Even not in our test suite? That would be worse.

I'm also for documentation change...<br/>
Just because this behaviour (first is just an index without last) is at least since 8.5 (if not earlier), so even if actual docu says something different, the code works as it works everywhere dozen years and there is no one reason to change the behaviour of tcl for the sake of documentation.

Especially because it is pretty simple to force the change case to the end, with supplying the `last` as `end` (to cause "documented" behaviour):

```
% string tolower ABCDE 2
ABcDE
% string tolower ABCDE 2 end
ABcde
```

@Jan, by the way, you wrote the same statement (result) for both variants, so here again (and a bit more clear example):

```
# if the code wins:
% string tolower ABCDE 2
ABcDE
  \------- only given index

# if the docu wins:
% string tolower ABCDE 2
ABcde
  \-/ ---- from given index to end
```

Commit [83bd3ccec5ec1208] contains both the documentation change (which adapts the documentation to the current behavior) and the implementation change (which adapts the implementation to the current documentation.

Please review: Should "string tolower ABC 1" result in "Abc" (as it currently does) or "Abc" (as the documentation suggests)?

I couldn't find any code which uses "string tolower/toupper/totitle", specifying _first_ but not _last_. But changing the implementation is a risk. Therefore I'm leaning towards changing the documentation. But if you don't agree with that (or if you agree with that), please let me know!