[xsl] Each value in an XPath array is a sequence of elements ... how to access a particular element in a particular array value?

["Costello, Roger L. costello@xxxxxxxxx" <xsl-list-service@xxxxxxxxxxxxxxxxxxxxxx>]

Hi Folks,

This is an extension to a question that I had last week ...

Recall that I want to group the <row> elements in the following XML. Each
group should consist of those <row> elements that have the same ARPT__IDENT,
TRM__IDENT values.

<Test>
    <row>
        <ARPT__IDENT>A</ARPT__IDENT>
        <TRM__IDENT>X</TRM__IDENT>
        <Data>Foo</Data>
    </row>
    <row>
        <ARPT__IDENT>A</ARPT__IDENT>
        <TRM__IDENT>X</TRM__IDENT>
        <Data>Bar</Data>
    </row>
    <row>
        <ARPT__IDENT>A</ARPT__IDENT>
        <TRM__IDENT>Y</TRM__IDENT>
        <Data>Blah</Data>
    </row>
    <row>
        <ARPT__IDENT>A</ARPT__IDENT>
        <TRM__IDENT>Y</TRM__IDENT>
        <Data>Plugh</Data>
    </row>
</Test>

That XML consists of two groups. Each group has two <row> elements.

Martin provided a great XPath 3.1 expression for doing the grouping:

<xsl:variable name="groups" as="array(element(row))*" select="
    let $keys :=
        distinct-values($rows/concat(ARPT__IDENT, '|', TRM__IDENT))
    return
        for $i in $keys
        return
            array {$rows[$i = concat(ARPT__IDENT, '|', TRM__IDENT)] }
    "/>

In the following, I select the first group:

<xsl:variable name="first-group" select="$groups[1]" />

Next, I want to select the <row> element in the first group that has Foo as
the value of the <Data> element. I thought that this would be the appropriate
XPath expression:

<xsl:variable name="Foo-row-first-group" select="$first-group/row[Data eq
'Foo']"/>

Apparently it is not the appropriate XPath expression, because I get this
error message:

	Axis step child::element(Q{}row) cannot
 	be used here: the context item is not a node

What is the appropriate XPath expression, please?

/Roger