max() of three xsl:number results

Hi

I have a <document> which consists of arbitrarily nested <section>s of mixed
content. Scattered throughout my document I have <target> elements. I am
trying to generate a listing which maps each <target> to its nearest
<section>:
a) if a target immediately precedes a section then I want that section's
location, otherwise
b) I want the closest predecessor section, which may be
 1) the section containing the target, or
 2) the nearest section which precedes it.

In the following simple example I have given each section a title which
reflects its location in the structure, and given each target a label which
does the same.

<!-- ===== XML ===== -->
<document>
 <section title="1">
  <section title="1.1">
   <section title="1.1.1">
    <section title="1.1.1.1">
    </section>
    <target label="1.1.1.1a" />
   </section>
   <target label="1.1.1.1b" />
  </section>
  <target label="1.2" />
  <section title="1.2">
  </section>
  <section title="1.3">
   <target label="1.3" />
  </section>
  <para />
 </section>
 <section title="2">
  <target label="2" />
  <para />
 </section>
 <section title="3">
 </section>
 <target label="3" />
</document>
<!-- ========== -->

The following stylesheet arrives at the numbers I want, viz:

<!-- ===== XSL 2.0 ===== -->
<xsl:stylesheet version="2.0"
   xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:output method="text" />

<xsl:template match="/">
<xsl:apply-templates />
</xsl:template>

<xsl:template match="target">
 <xsl:variable name="X">
  <xsl:number count="section" level="multiple" />
 </xsl:variable>
 <xsl:variable name="Y">
  <xsl:number select="preceding::section[1]"
     count="section" level="multiple" />
 </xsl:variable>
 <xsl:variable name="Z">
  <xsl:choose>
   <xsl:when test="following::*[1][self::section]">
    <xsl:number select="following::*[1][self::section]"
       count="section" level="multiple" />
   </xsl:when>
   <xsl:otherwise>
    <xsl:value-of select="0" />
   </xsl:otherwise>
  </xsl:choose>
 </xsl:variable>

 <xsl:value-of select="@label" />
 <xsl:text> X</xsl:text><xsl:value-of select="$X">
 <xsl:text> Y</xsl:text><xsl:value-of select="$Y">
 <xsl:text> Z</xsl:text><xsl:value-of select="$Z">
 <xsl:text>&#xa;</xsl:text>
</xsl:template>
<!-- ========== -->

When I apply this to the example XML above I get:

1.1.1.1a  X1.1.1  Y1.1.1.1  Z0
1.1.1.1b  X1.1    Y1.1.1.1  Z0
1.2       X1      Y1.1.1.1  Z1.2
1.3       X1.3    Y1.2      Z0
2         X2      Y1.3      Z0
3         X       Y3        Z0

Clearly what I want is the (lexically) greatest value out of X, Y and Z.
But when I replace the target template's output with the following lines:

<xsl:value-of select="@label" />
<xsl:text> max </xsl:text>
<xsl:value-of select="max(($X,$Y,$Z))" />
<xsl:text>&#xa;</xsl:text>

I get an error:

Error in XPath 2.0 expression Invalid lexical value - '1.1.1'

What am I doing wrong?

Thanks
Trevor