title: Can XSD choice be empty?
body: <p>I've read the XSD specification but I'm still confused. Is an empty choice valid in XSD? I mean, with a simple choice with no attributes declared, is mandatory to have either element "a" or element "b", or can it be empty?</p>
  
  <pre><code>&lt;xs:complexType name="ChoiceType"&gt;
      &lt;xs:choice&gt;
          &lt;xs:element name="a" type="type_a"/&gt;
          &lt;xs:element name="b" type="type_b"/&gt;
      &lt;/xs:choice&gt;
  &lt;/xs:complexType&gt;
  </code></pre>