Alignment of part in toc ( fo )

Hello,

I try to generate a pdf on a docbook book with xsl and
fop. I have a problem in formatting toc: the alignment
of the part isn't what I want...
Here is the result I have:
TOC
        Part1 ...
   Chapter1 ...
        Sect1 ...
        Sect1 ...
   Chapter2
        Sect1 ...
        Sect1 ...
        Part2 ...
   Chapter3 ...
        Sect1 ...

and I want
TOC
 Part1 ...
   Chapter1 ...
        Sect1 ...
        Sect1 ...
   Chapter2
        Sect1 ...
        Sect1 ...
 Part2 ...
   Chapter3 ...
        Sect1 ...

How can I configure this?
Thanks for your help.
Thierry 

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