M.Kay XSLT book, p. 120 Curly braces question

[Gert Bultman <g dot w dot bultman at ITS dot TUDelft dot nl>]

On page 120 of his XSLT book, Michael Kay describes where the curly
braces construct can and cannot be used. In the example at the top of
the page he describes a situation where it cannot be used, but fails to
give an alternative solution. 
I am at the moment facing a similar problem, where I would like to use a
variable or parameter as part of an XPath expression. However, whereas I
can use variables in constructions such as
$doc/FeatureSet[$i]/Feature[$j], where $doc is a nodeset generated by 

<xsl:variable name="doc" select="document(@FeatureFile)"/>

I cannot use a construction like $doc//$mypointer, where $mypointer has
the value 'FeatureSet[2]/Feature[4]'. (saxon & xt both balk at the
second $ sign in the expression)

To illustrate my point I'll include my xml and xsl fragments:

XML:

In my xml file which describes my data, I want to include a reference to
the original data that was used in the calculation. Since that data,
too, is stored in an XML file, I would ideally store an xpointer in one
of the attributes, and use that directly in my XSL stylesheet. 
Failing to do so, I then started using an additional attribute for
storing the counters. These I can use easily in my stylesheet, as the
example shows. 

<EventNode Id='1' FeatureFile='f1.xml' Fid="1.2"
FeatureId='FeatureSet[1]/Feature[2]' EventIn='CONT' EventOut='CONT'/>
<EventNode Id='2' FeatureFile='f1.xml' Fid="1.1"
FeatureId='FeatureSet[1]/Feature[1]' EventIn='CONT' EventOut='CONT'/>

XSL:

I use the document functiont to retrieve the external document (f1.xml)
as a nodeset, and I can then use constructions such as: 

<xsl:template match="EventNode">
<li>
Hello World : <xsl:value-of select="@Id"/>
<xsl:value-of select="@EventIn"/>
<xsl:value-of select="@EventOut"/>
<xsl:variable name="doc" select="document(@FeatureFile)"/>
<xsl:apply-templates select="$doc//FeatureSet[1]/Feature[1]"/>
</li>
</xsl:template>

(the rest of the stylesheet of course has appropriate templates for the
FeatureSet & Feature tags)
or even: 

<xsl:apply-templates
select="$doc/FeatureSet[substring-before($Fid,'.')]/Feature[substring-after($Fid,'.')]"/>


However, I would like to be able to use @FeatureId directly: 

<xsl:apply-templates select="document(@FeatureFile)//{@FeatureId}"/>

Or even by catching the xpointer in a variable first and then using a
concat: 

<xsl:variable name="doc" select="document(@FeatureFile)"/>
<xsl:variable name="pointer" select="@FeatureId"/>

<xsl:variable name="pointertwo"
select="concat('$doc','//',@FeatureId)"/>
or 
<xsl:variable name="pointertwo" select="concat('$doc','//',$pointer)"/>

<xsl:apply-templates select="$pointertwo"/>

=====
Any and all help will be greatly appreciated,
Gert Bultman
Delft Technical University
Computer Graphics Dept.


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