title: Define constants in XML
body: <p>How do I define constants in a XML document, for example</p>
  
  <pre><code>&lt;?xml version="1.0"?&gt;
  &lt;City&gt;
     &lt;Name&gt;Seattle&lt;/Name&gt;
     &lt;State&gt;WA&lt;/State&gt;
  &lt;/City&gt;
  
  &lt;City&gt;
     &lt;Name&gt;Tacoma&lt;/Name&gt;
     &lt;State&gt;WA&lt;/State&gt;
  &lt;/City&gt;
  </code></pre>
  
  <p>How do I define a variable like "State=WA" and use it in the  element as the value?</p>