Return an array from a function in XSLT

test.xsl

<?xml version="1.0" encoding="utf-8"?>
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
                xmlns:array="http://www.w3.org/2005/xpath-functions/array"
                xmlns:f="https://nwalsh.com/ns/functions"
                xmlns:map="http://www.w3.org/2005/xpath-functions/map"
                xmlns:xs="http://www.w3.org/2001/XMLSchema"
                exclude-result-prefixes="array f map xs"
                version="3.0">

<xsl:output method="xml" encoding="utf-8" indent="no"/>

<xsl:template match="/">
  <doc>
    <xsl:sequence select="count(array:flatten(f:test()))"/>
  </doc>
</xsl:template>

<xsl:function name="f:test" as="array(*)">
  <xsl:sequence select="array { (1,2,3) }"/>
</xsl:function>

</xsl:stylesheet>