Default values of parameters

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

            <xsl:template  match="/aaa">
                  <z-1>
                        <xsl:apply-templates  select="bbb"/>
                  </z-1>
                  <z-2>
                        <xsl:apply-templates  select="bbb">
                              <xsl:with-param  name="A"
                                          select="1"/>
                              <xsl:with-param  name="B">Bye</xsl:with-param>
                        </xsl:apply-templates>
                  </z-2>
                  <z-3>
                        <xsl:apply-templates  select="bbb">
                              <xsl:with-param  name="A"
                                          select="2"/>
                        </xsl:apply-templates>
                  </z-3>
            </xsl:template>

            <xsl:template  match="bbb">
                  <xsl:param  name="A"
                              select="999"/>
                  <xsl:param  name="B">Hello</xsl:param>
                  <xxx>
                        <xsl:value-of  select="$A"/>
                  </xxx>
                  <yyy>
                        <xsl:value-of  select="$B"/>
                  </yyy>
            </xsl:template>

      </xsl:stylesheet>

      <aaa>
            <bbb/>
      </aaa>

      <z-1>
            <xxx>999</xxx>
            <yyy>Hello</yyy>
      </z-1>
      <z-2>
            <xxx>1</xxx>
            <yyy>Bye</yyy>
      </z-2>
      <z-3>
            <xxx>2</xxx>
            <yyy>Hello</yyy>
      </z-3>