<element name="number" xmlns="http://relaxng.org/ns/structure/1.0">
   <attribute name="val">
      <list>
         <oneOrMore>
            <choice>
               <value>1</value>
               <value>2</value>
               <value>3</value>
            </choice>
         </oneOrMore>
      </list>
   </attribute>
   <text/>
</element>