How to get an apostrophe into a test expression

Suppose I have the following XML file a.xml 

<doc>
<a>le ciel</a>
<a>la maison</a>
<a>l'algue</a>
<a>l'est</a>
<a>les enfants</a>
<a>Norma</a>
</doc>

I want to transform this into the partitif article according to
French grammar: de le --> du, de la --> de la , de l' --> de l'
de les --> des; nothing --> de.

So I want to test on the presence of the string "le ", "la ", 
"l'", "les" and the rest to decide what to write. I use therefor the
following xsl stylesheet. However, I cannot find a way to "escape" or
"hide" the literal apostrophe on line 24. 

   <xsl:when test="substring($Nom,1,2) = 'l&apos;'">  

I have also tried to put the character reference on the same line:

   <xsl:when test="substring($Nom,1,2) = 'l&#39;'">  

but xt recognizes this character as a ', so the following character
gives problems. 

a.xsl:24: missing quote

I also tried 'l''' (as works in some languages), but the I get 

a.xsl:24: unexpected token

How can I refer to a quote inside a quoted string inside another quoted
string?

The complete xsl file follows.

<?xml version='1.0' encoding="ISO-8859-1"?>
<!DOCTYPE xsl:stylesheet [
<!ENTITY apos "&#39;">
]>
<xsl:stylesheet version="1.0"
     xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:output method="text" encoding="ISO-8859-1"/>

<xsl:template match="//a">
 <xsl:variable name="Nom" select="string(.)"/>
  <xsl:choose>
   <xsl:when test="substring($Nom,1,3) = 'le '">
    <xsl:text>du </xsl:text>
    <xsl:value-of select="substring-after($Nom,' ')"/>
   </xsl:when>
   <xsl:when test="substring($Nom,1,3) = 'la '">
    <xsl:text>de </xsl:text>
    <xsl:value-of select="$Nom"/>
   </xsl:when>
   <xsl:when test="substring($Nom,1,3) = 'les'">
    <xsl:text>des </xsl:text>
    <xsl:value-of select="$Nom"/>
   </xsl:when>
   <xsl:when test="substring($Nom,1,2) = 'l&apos;'">
    <xsl:text> de </xsl:text>
    <xsl:value-of select="$Nom"/>
   </xsl:when>
   <xsl:otherwise>
    <xsl:text> est la capitale de </xsl:text>
    <xsl:value-of select="$Nom"/>
   </xsl:otherwise>
  </xsl:choose>
  <xsl:text>.&#xA;</xsl:text><!-- retour à la ligne -->
</xsl:template>

</xsl:stylesheet>

Thanks for helping me solve this problem. mg


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