title: XML schema construct for "any one or more of these elements but must be at least one"
body: <p>I'm trying to set up part of a schema that's like a "Sequence" where all child elements are optional, but at least one of the elements <strong>must</strong> be present, and there could be more than one of them.</p>
  
  <p>I tried doing the following, but XMLSpy complains that "The content model contains the elements &lt;element name="DateConstant"&gt; and &lt;element name="DateConstant"&gt; which cannot be uniquely determined.":</p>
  
  <pre><code>    &lt;xs:choice&gt;
          &lt;xs:sequence&gt;
              &lt;xs:element name="DateConstant"/&gt;
              &lt;xs:element name="TimeConstant"/&gt;
          &lt;/xs:sequence&gt;
          &lt;xs:element name="DateConstant"/&gt;
          &lt;xs:element name="TimeConstant"/&gt;
      &lt;/xs:choice&gt;
  </code></pre>
  
  <p>Can this be done (and if so, how)?</p>
  
  <p>Some clarification:
  I only want to allow one of each element of the same name. There can be one "DateConstant" and/or one "TimeConstant", but not two of either.
  Gizmo's answer matches my requirements, but it's impractical for a larger number of elements.
  Hurst's answer allows two or more elements of the same name, which I don't want.</p>