sible to represent this constraint in XML
DTDs. XHTML Modularization takes the approach of partially
representing this restriction by, for example, disallowing an
<code>a</code> element from having an <code>a</code> element as a
<em>child</em>, without disallowing it from having an <code>a</code>
element as a non-immediate descendant.  This partial restriction has a
substantial cost in increasing the complexity and reducing the
modularity of the schemas. This implementation adopts a different
approa