Will every XSLT processor reduce upper-case('ß') to S

Hi Folks,

Here is a common German surname:

      Strau_

I used SAXON to convert the surname to uppercase:

    upper-case('Strau_')			returns STRAUSS

Notice that _ was converted to SS.

According to Wikipedia [1] there is ambiguity about whether _ should be
upcased to SS or S.

Will every XSLT processor reduce upper-case('_') to SS or will some reduce
upper-case('_') to S?

In other words, is this a "processor-specific" functionality?

/Roger

[1] http://en.wikipedia.org/wiki/Capital_%E1%BA%9E