I'm assuming you're getting Object: Stack error: should not be implemented in this class, use #basicNew instead
?
If so, then it looks like you need to add <shape: inherit>
in the body of your subclass.
See:
- Basic Smalltalk Subclass
- http://lists.gnu.org/archive/html/help-smalltalk/2010-06/msg00080.html
- http://www.gnu.org/software/smalltalk/manual/html_node/Inside-Arrays.html
That seems like a bit of a leaky abstraction to me - but I guess it is what it is.