[Zope] 'self' in external methods

[seb]

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.