Default values of function parameters

      <xsl:stylesheet
                  xmlns:my="myFunctions"
                  xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
                  version="2.0"
                  exclude-result-prefixes="my">
            <xsl:output  method="xml"
                        indent="yes"
                        omit-xml-declaration="yes"/>

            <xsl:function  name="my:multiply">
                  <xsl:param  name="a"/>
                  <xsl:param  name="b"
                              select="1"/>
                  <xsl:value-of  select="$a * $b"/>
            </xsl:function>

            <xsl:template  match="/aaa">
                  <xsl:value-of  select="my:multiply(4)"/>
            </xsl:template>

      </xsl:stylesheet>

      <aaa/>

Error: