I had to make one modification so that my @href attributes are correctly
resolved relative to the map, etc.:



<xsl:accumulator name="summary-data" initial-value="()" as="element()*">

        <xsl:accumulator-rule
match="*[@href[matches(.,'(dita(map)?|xml)$','i')]]">

            <xsl:sequence select="$value"/>

            <xsl:variable name="ref-path"
select="resolve-uri(@href,base-uri(.))"/>

            <xsl:if test="doc-available($ref-path)=true()">

                <xsl:apply-templates select="doc($ref-path)" mode="collect"/>

            </xsl:if>

        </xsl:accumulator-rule>

        <xsl:accumulator-rule match="tool" select="$value, ." />

        <xsl:accumulator-rule match="spare" select="$value, ." />

        <xsl:accumulator-rule match="supply" select="$value, ." />

    </xsl:accumulator>