I would be happy if I, from a given node, could count how many levels of 
  Y-nodes where below the current. I do not know if that was the answer that 
  David Carlisle gave.

yes.

<xsl:variable name="n">
 <xsl:for-each select=".//Y">
   <xsl:sort select="count(ancestor::Y)" datatype="number"/>
   <xsl:if test="position()=last()">
     <xsl:value-of select="count(ancestor::Y)"/>
   </xsl:if>
 </xsl:for-each>
</xsl:variable>

<xsl:value-of select="count(ancestor::Y) - $n"/>

is I believe the number you seek.

David

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