<schema xmlns=“www.ascc.net/xml/schematron” >

<pattern name="Open model">
     <rule context="AAA">
          <assert test="BBB"> BBB element is not present</assert>
          <assert test="CCC"> CCC element is not present</assert>
     </rule>
</pattern>
<pattern name="Closed model">
     <rule context="AAA">
          <assert test="BBB"> BBB element is not present</assert>
          <assert test="CCC"> CCC element is not present</assert>
          <assert test="count(BBB|CCC) = count (*)">There is an extra element</assert>
     </rule>
</pattern>

</schema>