Symphony.

<products>
    <section id="10" handle="products">Products</section>
    <entry id="42">
        <associated-office>
            <item id="43" handle="bolton-office" section-handle="offices" section-name="Offices">Bolton office</item>
        </associated-office>
        <name handle="pastures">Pastures</name>
    </entry>
</products>
<offices>
    <section id="9" handle="offices">Offices</section>
    <entry id="46">
        <office-name handle="bolton-office">Bolton office</office-name>
        <county handle="lancashire">Lancashire</county>
        <region>
            <item handle="north-west">North-West</item>
        </region>
    </entry>
</offices>
<regions>
    <section id="14" handle="regions">Regions</section>
    <entry id="78">
        <region handle="north-west">North-West</region>
    </entry>
</region>

<xsl:for-each select="regions/entry/region">
   <h2 id="{@handle}"><xsl:value-of select="." /></h2>
   <xsl:apply-templates select="/data/offices/entry[region/item/@handle = @handle]" />
</xsl:for-each>

<xsl:template match="offices/entry">
    <xsl:value-of select="office-name" />
    <xsl:apply-templates select="/data/products/entry[associated-office/item/@handle = office-name/@handle]" />
</xsl:template>

<xsl:template match="products/entry">
    <xsl:value-of select="name" />
</xsl:template>

<xsl:value-of select="/data/offices/entry[region/item/@handle = @handle]" />

<xsl:value-of select="/data/offices/entry[region/item[@handle = @handle]]" />

<xsl:for-each select="regions/entry/region">
    <h2 id="{@handle}"><xsl:value-of select="." /></h2>
    <xsl:apply-templates select="/data/offices/entry[region/item/@handle = @handle]" />
</xsl:for-each>

<xsl:for-each select="regions/entry/region">
    <xsl:variable name="currentregion" select="@handle"/>
    <h2 id="{@handle}"><xsl:value-of select="." /></h2>
    <xsl:apply-templates select="/data/offices/entry[region/item/@handle = $currentregion]" />
</xsl:for-each>

/data/offices/entry[region/item/@handle = @handle]