> <xsl:function name="f:index-of-node" as="xs:integer*">
>   <xsl:param name="nodes" as="node()*"/>
>   <xsl:param name="node" as="node()"/>
>   <xsl:for-each select="$nodes">
>     <xsl:sequence select="position()[current() is $node]"/>
>   </xsl:for-each>
> </xsl:function>
> 

or (perhaps less obscurely)

 <xsl:function name="f:index-of-node" as="xs:integer*">
   <xsl:param name="nodes" as="node()*"/>
   <xsl:param name="node" as="node()"/>
   <xsl:sequence select="1 to count($nodes)[subsequence($nodes, ., 1) is
$node]">
 </xsl:function>
 
 
 Regards,
 
 Michael Kay
 http://www.saxonica.com/
 http://twitter.com/michaelhkay