[ZDP] BackTalk to Document The Zope Book (2.5 Edition)/Variables and Advanced DTML

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A comment to the paragraph below was recently added via http://www.zope.org/Documentation/Books/ZopeBook/current/AdvDTML.stx#2-96

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    Like the *if* tag, calling the *unless* tag by name does existence
    checking, so::

      <dtml-unless the_easter_bunny>
        The Easter Bunny does not exist or is not true.
      </dtml-unless>

      % Anonymous User - Sep. 24, 2002 5:28 pm:
       This comes nice to provide the REQUEST with default variables
       (if they were not set in a previous html form) like

       <dtml-unless "REQUEST.has_key('the_easter_bunny')">
         <dtml-call "REQUEST.set('the_easter_bunny', 0)">
       </dtml-unless>