wicket/src/test/java/org/apache/wicket/markup/MarkupInheritanceExtension_4.html

extension: before (will be ignored)
<wicket:extend>
  <!-- note: extension and base can not have equals ids like e.g. "label" -->
  extension: 1 <span wicket:id="label3">Label 3</span> extension: 2
</wicket:extend>
extension: after (will be ignored)