title: Using + operator in XML DTD
body: <p>I have query regarding specifying sub-parts of an element while defining a DTD for XML. I want to have an element titled "Description", which may have any inter-leaved sequence of a BookRef and PCDATA. I'm using following statement in my XML DTD. </p>
  
  <pre><code>&lt;!ELEMENT Description (#PCDATA|Courseref)* &gt;
  </code></pre>
  
  <p>However, I want to enforce a more strict constraint than *. I want to use +, which should mandate the having of at least one PCDATA or Courseref. However, when I use + instead of *, I get a parse error using xmllint. </p>
  
  <p>I'm new to DTD and I want to know, if it is illegal as per XML DTD Specs to specify a + operator. </p>