<html xmlns:tal="http://xml.zope.org/namespaces/tal">
  <p tal:replace="structure string:&lt;a&gt;bar&lt;/a&gt;"
     tal:attributes="href string:http://www.python.org">dummy text</p>
  <p tal:define="x python:1" tal:on-error="string:bad boy!">
   <span tal:define="x python:2">
    <span tal:define="x python:3">
     <span tal:content="python:1/0"/>
    </span>
   </span>
  </p>
  <p tal:on-error="string:x undefined">
    <span tal:content="x"/>
  </p>
</html>
