Another mind-blowing solution. This one from Michael Kay:

<!--
    Determine if the ranges in $seq are consecutive.
    $seq = (0-2, 3-7, 8, 9-15) returns true
    $seq = (0-3, 3-7, 8, 9-15) returns false
-->
<xsl:function name="f:isConsecutive" as="xs:boolean">
    <xsl:param name="seq"/>

    <xsl:variable name="seq-with-to" as="xs:integer*">
        <xsl:evaluate xpath="$seq ! replace(., '-', ' to ') =>
string-join(',')"/>
    </xsl:variable>
    <xsl:sequence select="deep-equal($seq-with-to, $seq-with-to[1] to
$seq-with-to[last()])"/>
</xsl:function>