DOCBOOK: Line Numbering not working in <programlisting> tag.

Hi,

Using Dsssl 1.72, and xml 4.1.2, the programlisting tag igores the 
linenumbering="numbered" attrib, and produces the same grey box in fixed 
width font (without line numbers) as if the attribute was not specified.

Example use:
<sect1>
  <title>This is a Title</title>
  <para>
   This is a paragraph, listing follows.
  </para>
  <programlisting linenumbering="numbered">
Listing line 1
line 2
3rd line
etc.
  </programlisting>
</sect1>

Perhaps someone can see what I'm doing wrong.

Jonathan