<schema xmlns="http://www.ascc.net/xml/schematron" >
     <pattern name="Attributes present">
          <rule context="BBB">
               <assert test="not(@aaa) or (@aaa and @bbb)">The element must not have an isolated aaa attribute</assert>
               <assert test="not(@bbb) or (@aaa and @bbb)">The element must not have an isolated bbb attribute</assert>
          </rule>
     </pattern>
</schema>