As mentioned earlier, named patterns are the only way to represent recursive models. We don’t yet have all the building blocks needed to define a recursive XHTML div element, for example, but let’s start with a simpler example. If our library is divided into categories, each having a title, zero or more embedded category elements, and zero or more books, you can write (assuming that named patterns have been defined for the book element):

<define name="category">
 <element name="category">
  <element name="title">
   <text/>
  </element>
  <zeroOrMore>
   <ref name="category"/>
  </zeroOrMore>
  <zeroOrMore>
   <ref name="book"/>
  </zeroOrMore>
 </element>
</define>

or:

category = element category{
 element title{text},
 category *,
 book*
}

Note that in this case, the recursive reference to the category named pattern must be optional. Otherwise the document is required to have an infinite depth!