Token a string with spaces into Nested-List

<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
    xmlns:xs="http://www.w3.org/2001/XMLSchema" xmlns:vis="vishFunction"
    exclude-result-prefixes="xs vis"
    version="2.0" >
    <xsl:variable name="assets" select="'123 234 456 789 0001'"/>
    <xsl:output method="html"></xsl:output>
    <xsl:template match="/">
        <xsl:sequence select="vis:nestedList($assets)"/>
    </xsl:template>
   
    <xsl:function name="vis:nestedList">
        <xsl:param name="assetID"/>
        <ul>
            <li>
                <xsl:variable name="firstToken" select="tokenize(normalize-space($assetID),' ')[1]"/>
                <xsl:value-of select="$firstToken"/>
                <xsl:variable name="restAssets" select="normalize-space(substring-after($assetID,$firstToken))"/>
                <xsl:if test="string-length(normalize-space($restAssets)) != 0">
                    <xsl:message><xsl:value-of select="$restAssets"/></xsl:message>
                <xsl:sequence select="vis:nestedList($restAssets)"/>
                </xsl:if>
            </li>
        </ul>
    </xsl:function>
</xsl:stylesheet>