out that it's actually quite likely that a string that is an extension of
the given prefix will sort as larger than the "greater" string our previous
code created.  To provide some defense against that, do the comparisons
against a modified string instead of just the bare prefix.  We tack on
"Z", "z", "y", or "9", whichever is seen as largest in the current locale.
Testing suggests that this is sufficient at least for cases involving
ASCII data.