[Zope] 'self' in external methods

Hello, Whenever I do a

def play(self,sound): return sound

and a

<dtml-var "play('plonk')">

I get a

Error Type: TypeError Error Value: not enough arguments; expected 2, got 1

Now, I thought 'self' was passed _implicitly_, as explained in http://www.zope.org/Documentation/How-To/ExternalMethods, but I'm actually having to do a

<dtml-var "play(this(),'plonk')">

to pass the context to 'self' in the External method. Explanations gratefully received. Thanks, seb.