base: before
  <!-- note: extension and base can not have equals ids like e.g. "label" -->
  extension: 1 <span wicket:id="label1">base label 1</span> extension: 2
  <wicket:child><wicket:extend>
  <!-- note: extension and base can not have equals ids like e.g. "label" -->
  extension: 1 <span wicket:id="label3">extension label</span> extension: 2
</wicket:extend></wicket:child>
  extension: 1 <span wicket:id="label2">base label 2</span> extension: 2
  base: after