when I'm processing the <article>s, I need to do something different with
the first and last, that is:

If you run into difficulties while applying
Peter's suggestion about using position()=1
etc., check whether using the preceding-sibling
and following-siblig axes could help.
Test whether there is nor article before:
 <xsl:if test="not(preceding-siblinc::article)">
Test whether there is no element (whether article
or something else) before:
 <xsl:if test="not(preceding-siblinc::*)">
Test whether there is no node before:
 <xsl:if test="not(preceding-siblinc::node())">
Test whether there is no node before except
text nodes contraining only white space:
 <xsl:if test="not(preceding-siblinc::node()[normalize-space()])">