In section 16.2.5 fn:for-each-pair:

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

It's missing the close quote for the first two parameters.

Thank you, now fixed.