[xsl] xsl:number

["Fraser Goffin" <goffinf@xxxxxxxxxxx>]

In a stylesheet I am storing the value returned by <xsl:number> in a 
variable and then using that variable within an XPath expression, thus :-

<xsl:template match="foo">
   <xsl:variable name="instance">
       <xsl:number/>
   </xsl:variable>
   <xsl:variable name="instanceNo" select="number($instance)"/>

   <xsl:choose>
       <xsl:when test="//someXPath/Condition[$instanceNo]/ItemType/Code) = 
'some value'"/>
       ...

Is it possible to optimise the assignment to the variable instanceNo to do 
it in 1 step (rather than using 2 variables) as part of the select ?

I've tried various combinations but can't seem to find one that works and 
I'm wondering whether because <xsl:number/> is an instruction it can't be 
used in that way ??

Fraser.