title: Xml schema - define child elements 0-* in any order
body: <p>I want to define a xml schema where the element Connectors have 0-* child elements. Either Sequence, Association or Message in any order and 0 to many times. I.e.</p>
  
  <pre><code>&lt;Connectors&gt;
      &lt;Sequence /&gt;
      &lt;Association /&gt;
      &lt;Message /&gt;
      &lt;Sequence /&gt;
      &lt;Sequence /&gt;
      &lt;Message /&gt;
      &lt;Message /&gt;
      &lt;Association /&gt;
  &lt;/Connectors&gt;
  </code></pre>
  
  <p>I tried to define the following schema but it seems like the order is fixed.</p>
  
  <pre><code>&lt;xs:element name="Connectors"&gt;
      &lt;xs:complexType&gt;
          &lt;xs:sequence&gt;
              &lt;xs:element ref="Association" minOccurs="0" maxOccurs="unbounded" /&gt;
              &lt;xs:element ref="Message" minOccurs="0" maxOccurs="unbounded" /&gt;
              &lt;xs:element ref="Sequence" minOccurs="0" maxOccurs="unbounded" /&gt;
          &lt;/xs:sequence&gt;
      &lt;/xs:complexType&gt;
  &lt;/xs:element&gt;
  </code></pre>