libxml2/test/schematron/zvon2.sct

<schema xmlns="http://www.ascc.net/xml/schematron" >
     <pattern name="Character @ forbidden">
          <rule context="*">
               <report test="contains(.,'@')">Text in element
                    <name/> must not contain character @
               </report>
          </rule>
     </pattern>
</schema>