how do I get around "Axis steps" created by "atomic values" in XSL 2.0?

Question

In XSL 2.0, I'm trying to iterate through some data by the distinct values, and then do something with them.

<xsl:for-each select="distinct-values(InvoiceLine/Service/ServiceMnemonicCode)">

    <xsl:variable name="mnemonic">
     <xsl:value-of select="."/>
    </xsl:variable>

    <fo:table-row>
     <fo:table-cell>
      <fo:block>
       <xsl:value-of select="InvoiceLine/Service[ServiceMnemonic=$mnemonic]/ServiceDescription"/>

      </fo:block>
     </fo:table-cell>
    </fo:table-row>
   </xsl:for-each>

However I end up with the following error:

XPTY0020: Axis step
 child::element({http://schemas.blabla.com/etp/invoice/types}InvoiceLine, xs:anyType)
 cannot be used here: the context item is an atomic value
ailed to compile stylesheet. 1 error detected.

I've bee googling furiously, and I do see people complaining about "atomic values" but I haven't seen anyone suggest what to do about it. I've using Saxon9. Any insight would be greatly appreciated.

Answer 1

Don't know if its the best solution, but this seems to work:

<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>