<schema xmlns="http://www.ascc.net/xml/schematron" >
     <pattern name="Test attribute">
          <rule context="CCC">
               <report test="parent::BBB and not(@id)">Attribute id is missing</report>
               <report test="not(parent::BBB) and @id">Attribute id is used in wrong context</report>
          </rule>
     </pattern>
</schema>

