condition: Conditionally insert or remove an element
  
  Syntax

    'tal:condition' syntax::

      argument ::= expression

  Description

    The 'tal:condition' statement includes the statement element in the
    template only if the condition is met, and omits it otherwise.  If
    its expression evaluates to a *true* value, then normal processing
    of the element continues, otherwise the statement element is
    immediately removed from the template.  For these purposes, the
    value *nothing* is false, and *default* has the same effect as
    returning a true value.

    *Note: Zope considers missing variables, None, zero, empty strings,
    and empty sequences false; all other values are true.*

  Examples

    Test a variable before inserting it (the first example tests for
    existence and truth, while the second only tests for existence)::

      <p tal:condition="request/message | nothing"
	 tal:content="request/message">message goes here</p>

      <p tal:condition="exists:request/message"
	 tal:content="request/message">message goes here</p>

    Test for alternate conditions::

      <div tal:repeat="item python:range(10)">
	<p tal:condition="repeat/item/even">Even</p>
	<p tal:condition="repeat/item/odd">Odd</p>
      </div>