[Zope] External Method - "context" arg

The Zope book chapter "Advanced Zope Scripting" shows a method with an arg list like this: def makeThumbnail(context, original_id, size=128): When I create a method with the "context" arg, Zope complains that the parameter "context" was omitted from the request. THE REAL QUESTION::: Using a Python Method, it's easy to get the variables "context" and "travers_subpath." How do I get these two values when using an External Method. Thanks for any assistance. Tom