getAppStructureById?

Lofton,

I'm struggling with the definition of the getAppStructureById. In the spec 
it indicates that getAppStructureById returns a WebCGMNode. 

But the examples and tests imply that it is returning a WebCGMAppStructure
object, e.g.
 
http://www.w3.org/Submission/2006/SUBM-WebCGM20-20060313/WebCGM20-DOM.html#L5095
In this example it is using pic.getAppstructure("A"); to get
"obj" floowed by obj.setAppStructureAttr. But if "obj" is of type 
WebCGMNode, it doesn't have a setAppStructureAttr member?

I understand that WebCGMnode is the base of  the WebCGMAppStructure 
interface and so inherits the attributes of WebCGMNode. But WebCGMNode 
doesn't inherit from WebCGMAppStructure. 

So What am I missing here?

Regards, 
Don.