title: XSD - Override element
body: <p>I've a base class, marked as abstract:</p>
  
  <pre><code>&lt;!-- TYPE: BASE CLASS --&gt;
  &lt;xs:complexType name="BaseClass" abstract="true"&gt;
      &lt;xs:sequence&gt;
          &lt;xs:element name="name" type="xs:string" minOccurs="1" maxOccurs="1"/&gt;
          &lt;xs:element name="implementation" type="BaseClass" minOccurs="0" maxOccurs="1"/&gt;
      &lt;/xs:sequence&gt;
  &lt;/xs:complexType&gt;
  </code></pre>
  
  <p>Now the class that inherits BaseClass and overrides 'implementation' with null:</p>
  
  <pre><code>&lt;xs:complexType name="MyClass"&gt;
      &lt;xs:complexContent&gt;
          &lt;xs:extension base="BaseClass"/&gt;
      &lt;/xs:complexContent&gt;
  &lt;/xs:complexType&gt;
  </code></pre>
  
  <p>I've tried this but is invalid:</p>
  
  <pre><code>&lt;xs:complexType name="MyClass"&gt;
      &lt;xs:complexContent&gt;
          &lt;xs:extension base="BaseClass"&gt;
              &lt;xs:sequence&gt;
                  &lt;xs:element name="implementation" fixed="xs:nil"/&gt;
              &lt;/xs:sequence&gt;
          &lt;/xs:extension&gt;
      &lt;/xs:complexContent&gt;
  &lt;/xs:complexType&gt;
  </code></pre>
  
  <p>Is there a way to do this?</p>