Counting level of nodes beneath current (in XPATH)

X -> Y+
Y -> Z X?
Z -> (textnode)

An example is like this:
<X>
 <Y>
   <Z>lv 1</Z>
   <X>
     <Y>
       <Z>lv 2</Z>
     </Y>
   </X>
 </Y>
 <Y>
   <Z>lv 1</Z>
   <X>
     <Y>
       <Z>lv 2</Z>
       <X>
         <Y>
           <Z>lv 3</Z>
         </Y>
       </X>
     </Y>
     <Y>
       <Z>lv 2</Z>
       <X>
         <Y>
           <Z>lv 3</Z>
         </Y>
       </X>
       <X>
         <Y>
           <Z>lv 3</Z>
           <X>
             <Y>
               <Z>lv 4</Z>
             </Y>
             <Y>
               <Z>lv 4</Z>
             </Y>
           </X>
         </Y>
       </X>
     </Y>
   </X>
 </Y>
</X>

<xsl:template match="/">
 <xsl:choose>
   <xsl:when test="$lv='1'">
     <xsl:apply-templates select="Y[not(X)]" mode="One"/>
   </xsl:when>
   <xsl:when test="$lv='2'">
     <xsl:apply-templates select="Y[not(X/Y/X)]" mode="Two"/>
   </xsl:when>
   ...
   <xsl:otherwise>
     <xsl:apply-templates select="Y" mode="Zero"/>
   </xsl:otherwise>
 </xsl:choose>
</xsl:template>

<xsl:template match="Y" mode="One">
 <!-- Do some stuff -->
 <xsl:apply-templates select="Y[not(X)]" mode="One"/>
</xsl:template>

Thank you!
Ragulf Pickaxe :-|