XPath/XQuery for-each-pair function

Applies the function item $action to successive pairs of items taken one from $seq1 and one from $seq2, returning the concatenation of the resulting sequences in order.

The effect of the function is equivalent to the following implementation in XQuery:

declare function fn:for-each-pair($seq1, $seq2, $action)
{
   if(fn:exists($seq1) and fn:exists($seq2)) 
   then (
     $action(fn:head($seq1), fn:head($seq2)),
     fn:for-each-pair(fn:tail($seq1), fn:tail($seq2), $action)
   )
   else ()
};

or its equivalent in XSLT:

<xsl:function name="fn:for-each-pair">
  <xsl:param name="seq1"/>
  <xsl:param name="seq2"/>
  <xsl:param name="action"/>
  <xsl:if test="fn:exists($seq1) and fn:exists($seq2)">
    <xsl:sequence select="$action(fn:head($seq1), fn:head($seq2))"/>
    <xsl:sequence select="fn:for-each-pair(fn:tail($seq1), fn:tail($seq2), $action)"/>
  </xsl:if>
</xsl:function>