<schema xmlns="http://www.ascc.net/xml/schematron" >
     <pattern name="Predefined values">
          <rule context="AAA">
               <report test="normalize-space(.) = 'aaa' or normalize-space(.) = 'bbb'">O.K.</report>
               <assert test="normalize-space(.) = 'aaa' or normalize-space(.) = 'bbb'">The AAA value is not permitted.</assert>
          </rule>
     </pattern>
</schema>