[Zope] Acquisition confusion: more info

In a simplified, contrived system, I have DTML Documents 'foo', 'bar' and 'spam'. The entire contents of these files are: foo = '<!--#var bar-->' bar = '<!--#var spam-->' spam = 'This is Spam!' This example works as expected. That is, the document foo contains 'This is Spam!'. Changing foo to be '<!--#var "bar()"-->' results in a KeyError on the name "spam". I get the same results if I use DTML Methods. What do I have to do to get "bar()" to work? Richard