Treatment of tabs in programlisting in FO output

In a <programlisting> I include a Makefile by

<programlisting><textobject><textdata 
fileref="Makefile"/></textobject></programlisting>

which contains the usual tabs in the commands of a rule.

Both in HTML and FO output, these tabs are preserved.

This looks fine in HTML because they are surreounded by a <pre>.

But XEP converts these tabs inside the fo:block into only one whitespace 
character.

Is this correct? So I have to substitute the tabs by 8 white spaces 
myself? Or is it an error of XEP (or the stylesheets)?


Joachim