title: Retrieving all attribute names of an XML element with xpath on xmlstarlet
body: <p>I can see how I can retrieve all attribute values:<br>
  <code>xml sel -t -v "//element/@*"</code></p>
  
  <p>but I want to get all attribute <em>names</em>.<br>
  I can get the n'th name by, for example <code>xml sel -t -v "name(//x:mem/@*[3])"</code> which returns the 3rd attribute name.<br>
  but <code>xml sel -t -v "name(//x:mem/@*)"</code> does not work (returns the 1st attribute name only)...</p>
  
  <h2>Is there a way to get <em>all</em> attribute names?</h2>