<items>
<item id="1" parent_id="0">Шкаф</item>
<item id="2" parent_id="1">Полка</item>
<item id="3" parent_id="1">Полка</item>
<item id="4" parent_id="3">Книга</item>
<item id="5" parent_id="4">Страница</item>
...
</items>

<ul>
    <li>
        Шкаф
        <ul>
            <li>
                Полка
            </li>
            <li>
                Полка
                <ul>
                    <li>
                         Книга
                         <ul>
                              <li>
                                  Страница
                              <li>
                         </ul>
                    <li>
                </ul>
            </li>
    </li>
</ul>

<xsl:template name="ctx1">
	<xsl:choose>
		<xsl:when test="contains(substring-after($test1, '/'),'category')">
			<xsl:call-template name="ctx1">
				<xsl:with-param name="test1" select="substring-after($test1,'/')"/>
			</xsl:call-template>
		</xsl:when>
		<xsl:when test="not(contains($test1,'/'))">
			<xsl:call-template name="ctx2">
				<xsl:with-param name="test2" select="$test1"/>
			</xsl:call-template>
		</xsl:when>
		<xsl:otherwise>
			<xsl:call-template name="ctx2">
				<xsl:with-param name="test2" select="substring-before($test1,'/')"/>
			</xsl:call-template>
		</xsl:otherwise>
	</xsl:choose>
</xsl:template>

<xsl:template match="forum">
	<ul>
		<group-list>
			<xsl:apply-templates select="message[generate-id(.) = generate-id(key('parent_id',@parent_id))]"/>
		</group-list>
	</ul>
</xsl:template>
  
<xsl:template match="message">
	<li>
		<group name="{@parent_id}">
			<xsl:for-each select="key('parent_id',@parent_id)">
				<a href="forums/?id={@forum_id}&mid={@root_id}" style="color:#0000FF;">
					<xsl:value-of select="@subject"/>
				</a>
			</xsl:for-each>
		</group>
	</li>
</xsl:template>