Explicit raising of errors

      <xsl:stylesheet
                  xmlns:sch="http://www.w3.org/2001/XMLSchema"
                  xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
                  version="2.0"
                  exclude-result-prefixes=" sch">
            <xsl:output  method="xml"
                        indent="yes"
                        omit-xml-declaration="yes"/>

            <xsl:template  match="/aaa">
                  <xsl:choose>
                        <xsl:when  test=". != 12">
                              <xsl:copy-of  select="."/>
                        </xsl:when>
                        <xsl:otherwise>
                              <xsl:value-of  select="error(sch:QName('error9999'),'Oh, not again',.)"/>
                        </xsl:otherwise>
                  </xsl:choose>
            </xsl:template>

      </xsl:stylesheet>

      <aaa>12</aaa>

Error: