DOCBOOK-APPS: chapter numbering

Is chapters supposed to be counted per part ? I mean is this a bug or expected
behaviour that chapters are rendered with contignous numbering along whole
book without reset at begining of each part. I use following syntax:

<book>
  <part><title>Part</title>
    <chapter><title>Chapter</title>...</chapter>
    <chapter><title>Chapter</title>...</chapter>
  </part>
  <part><title>Part</title>
    <chapter><title>Chapter</title>...</chapter>
    <chapter><title>Chapter</title>...</chapter>
  </part>
</book>

and it is rendered as:

 I. Part 
   1. Chapter
   2. Chapter
II. Part
   3. Chapter
   4. Chapter

while I would expect

 I. Part 
   1. Chapter
   2. Chapter
II. Part
   1. Chapter
   2. Chapter

Did I missed something ? Is there a switch for switch this behaviour without
defining label="" for each chapter ?

I use docbook XSL 1.57.0 with xsltproc.

Wlodzimierz Skiba