If youve already made 

        <items>
                <item>V1</item> <!-- item 0 -->
                <item>V2</item> <!-- item 1 -->
                <item>V3</item> <!-- item 2 -->
        </items>
and put it in file1.xml then

                        <item><xsl:value-of
                        select="'missing-expression'"/></item>
is
                        <item><xsl:value-of select="count(doc('file1.xml')/items/item[.=current()]/preceding-sibling::*)"/></item>

as always, keys can be used to speed up this kind of expression.

David