Content models that are not regular expressions

The use of named model groups allows content models that are not regular
expressions.  For example:

  <s:group name="recur">
    <s:sequence>
      <s:element name="open"/>
      <s:group ref="recur" minOccurs="0" maxOccurs="unbounded"/>
      <s:element name="close"/>
    </s:sequence>
  </s:group>

Useful though this would be, it is probably not intended.

-- Richard