Re: configuring a conditional

Aside: I've still not worked out HOW to deal with this first/subsequent
business generally.

It's easy enough (although possibly expensive) to know if you are the
first citation to a particular reference, eg you could use xsl:number
and see if you get the number 1, or perhaps better havs an xsl:key that
matches all references, keyed on the reference id then you just have to
ask if the current node is the first node in document order in the
sequence returned by the key for this reference.
so for example if the syntax is

<xref linkend="foo"/>

and you want to know if this is the first such reference

<xsl:key name="xr" match="xref use="@linkend"/>
..
<xsl:template match="xref">

<xsl:if test=". is key('xr',@linkend)[1]">
 I'm first

    <xsl:param name="cite-ref" as="element()*"/>
    <xsl:message>
      <xsl:if test="$cite-ref is key('refs',@linkend)[1]">FIRST</xsl:if>
    </xsl:message>