Nesting in $bibliography.collection

I have just had a system break and discovered that it seems to be 
because I introduced bibliodiv elements into the document identified by 
$bibliography.collection. I am using docbook-xsl-1.72.0

 From inspection the XSL seems to back up this observation. It is 
looking for direct children of bibliography.

Is this intended? Am I missing something?

Nigel

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Nigel Hardy     Tel: +44 1970 622 434.   http://users.aber.ac.uk/nwh/
Dept. Computer Sci,  University of Wales, Aberystwyth, SY23 3DB, UK
Adran Cyfrifiadureg, Prifysgol Cymru,     Aberystwyth, SY23 3DB, UK