<xsl:for-each select="node">
  <xsl:sort select="index-of(('z', 'a', 'b'),@class)"/>


or


<xsl:for-each select="for $c in ('z', 'a', 'b') return node[@class=$c]">

David

________________________________________________________________________
The Numerical Algorithms Group Ltd is a company registered in England
and Wales with company number 1249803. The registered office is:
Wilkinson House, Jordan Hill Road, Oxford OX2 8DR, United Kingdom.

This e-mail has been scanned for all viruses by Star. The service is
powered by MessageLabs. 
________________________________________________________________________