Re: XPath expression confusion

> <xsl:variable name="all-reqs" select="document('requirements.xml')/requirements//requirement/number" />
> 
>   <xsl:template match="/">
>     <un-assigned-reqs>
>       <xsl:for-each select="$all-reqs">
>         <xsl:choose>
>           <xsl:when test="/system/components/component/requirement/@number = ." />

That test can never be true.
If the top level element in requirements.xml is requirements, then the
path starting /system will select the empty node set.

/ at the start of an xpath means the root of the tree containing the
current node, and the current node  here is an item in $all-reqs which
is a tree rooted in the node returned by the document() function.
I suspect you meant

 <xsl:variable name="all-reqs" select="document('requirements.xml')/requirements//requirement/number" />
<xsl:variable name="$root" select="/"/>
 
   <xsl:template match="/">
     <un-assigned-reqs>
       <xsl:for-each select="$all-reqs">
         <xsl:choose>
           <xsl:when test="$root/system/components/component/requirement/@number = ." />

although you dont need an xsl:choose with an empty branch it looks like
a single xpath is all you need

select="$all-reqs[not(.=$root/system/components/component/requirement/@number)]">
<requirement><xsl:value-of select="." /></requirement>

David

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