title: when to use a URN for xmlns
body: <p>For a "hello world" type xml document:</p>
  
  <pre><code>&lt;?xml version="1.0" encoding="UTF-8"?&gt;
  &lt;!-- see http://www.w3.org/TR/REC-xml-names/ --&gt;
  &lt;bk:book xmlns:bk="urn:loc.gov:books" xmlns:isbn="urn:ISBN:0-395-36341-6"&gt;
    &lt;bk:title&gt;Cheaper by the Dozen&lt;/bk:title&gt;
    &lt;isbn:number&gt;1568491379&lt;/isbn:number&gt;
  &lt;/bk:book&gt;
  </code></pre>
  
  <p>you can arbitrarily define urn's on the fly for custom purposes?</p>