David A. Black wrote:
> It's a class variable, which is actually a class-hierarchy variable
> (shared between a class and its descendants), and also visible to all
> the instances of all of those classes.
> 
> In other words, it's a kind of class-hierarchy-scoped global. Think of
> it as $$name :-)
> 
> 
> David

Ok thanks. That's why I always got error when access to this variable 
outside of the class-hierarchy.
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