<rule id="Number">
     <one-of>
         <item weight="2">
            <!-- Seis is weighted to be twice as likely to be -->
            <!-- recognized as Tres -->
            <ruleref uri="Seis" />
         </item>
         <item weight="1">
            <!-- Notice that the weight is applied in the item -->
            <!-- tag in grXML -->
            <ruleref uri="Tres" />
        </item>            
      </one-of>
   </rule>
   <rule id="Number">
      <one-of>
         <item>tres</item>
         <item>tre</item>
      </one-of>
   </rule>
   <rule id="Seis">
      <item>seis</item>
   </rule>