[ZDP] BackTalk to Document The Zope Book (2.5 Edition)/Using Zope Page Templates

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A comment to the paragraph below was recently added via http://www.zope.org/Documentation/Books/ZopeBook/current/ZPT.stx#2-46

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    Using Page Templates you can dynamically query your environment
    and selectively insert text depending on conditions. For example,
    you could display special information in response to a cookie::

      <p tal:condition="request/cookies/verbose | nothing">
        Here's the extra information you requested.
      </p>

      % Anonymous User - Sep. 10, 2002 3:22 pm:
       At what point would we be better off in Python?

      % Anonymous User - Jan. 14, 2003 6:47 pm:
       what is the value of 'nothing' ?? Is that a special variable and what is its value ?
       What is the meaning of '|' is zopesque for an 'OR' or an 'XOR' ??
       how is <p tal:condition="request/cookies/verbose | nothing">
       different from <p tal:condition="request/cookies/verbose"> ??