A comment to the paragraph below was recently added via http://www.zope.org/Documentation/Books/ZopeBook/current/AppendixC.stx#3-60
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Defining two variables, where the second depends on the first::
tal:define="mytitle template/title; tlen python:len(mytitle)"
% Anonymous User - Sep. 3, 2002 8:50 am:
ignore t by tlen