[xsl] XSLT 2 and XSLT 3: Best Way To Get Set of Unique Docs for A Set of Elements

["Eliot Kimber ekimber@xxxxxxxxxxxx" <xsl-list-service@xxxxxxxxxxxxxxxxxxxxxx>]

Given the variable $links that is a sequence of element()s where the elements
could be from different documents, what is the best way to get the set of
unique documents?

I need an XSLT 2 answer but an XSLT 3 answer would also be useful.

I feel like there's an obvious solution I'm overlooking but the only thing I
can think of is to compare the document-uri() values of all the elements:

    <xsl:variable name="topicURIs" as="xs:string*"
      select="distinct-values(for $e in $links return
document-uri(root($e)))"
    />
    <xsl:variable name="topics" as="document-node()*"
        select="
        for $uri in $topicURIs
        return root(($links[document-uri(root(.)) eq $uri])[1])
        "
    /> o;?

This works but seems unnecessarily complicated.

Thanks,

Eliot
--
Eliot Kimber
http://contrext.com