title: Element Declaration: Defining content as #PCDATA and one occurence of child element
body: <p>In the xml spec, it says:</p>
  
  <blockquote>[Definition: An element type has mixed content when elements of that type may contain character data, optionally interspersed with child elements.] In this case, the types of the child elements may be constrained, but not their order or their number of occurrences</blockquote>
  
  <p>Is there any way to work around this to make this xml valid:</p>
  
  <pre><code>&lt;parent&gt;
     A text node
     &lt;child/&gt;
  &lt;/parent&gt;
  </code></pre>
  
  <p>but this xml invalid:</p>
  
  <pre><code>&lt;parent&gt;
     A text node
     &lt;child/&gt;
     &lt;child/&gt;
  &lt;/parent&gt;
  </code></pre>
  
  <p>Also, do you know any reason for why this is not allowed?</p>
  
  <pre><code>&lt;!ELEMENT parent (#PCDATA,child)&gt;
  </code></pre>