IF in a variable declaration

<compte>
   <pdf>1</pdf>
   <pdf>3</pdf>
</compte>

               <xsl:variable name="present">
                   <xsl:for-each select="/liste/id/compte/pdf">
                       <xsl:if test="/liste/id/compte/pdf=$ID">O</xsl:if>
                   </xsl:for-each>
               </xsl:variable>
               <xsl:value-of select="$present"/>

Do you have any help please ?
I managed to do this when <pdf> tag enclude other tags or attribute.
I don't know how to write the test for itself.