为什么不能在这个xpath表达式中使用子元素进行导航?

不知道它是否是最佳解决方案,但这似乎有效:

<xsl:template match="Invoice">
        <xsl:variable name="invoice">
            <xsl:copy-of select="."/>
        </xsl:variable>
        <fo:table border="0.5pt solid black" text-align="center">
            <fo:table-body>
                <xsl:for-each select="distinct-values(InvoiceLine/Service/ServiceMnemonicCode)">
                    <xsl:sort/>
                    <xsl:variable name="code">
                        <xsl:copy-of select="."/>
                    </xsl:variable>
                    <fo:table-row>
                        <fo:table-cell>
                            <fo:block>
                                <xsl:value-of select="($invoice/node()/InvoiceLine/Service[ServiceMnemonicCode=$code]/ServiceDescription)[1]"/>
                            </fo:block>
                        </fo:table-cell>
                    </fo:table-row>
                </xsl:for-each>
            </fo:table-body>
        </fo:table>
    </xsl:template>