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Py_CLEAR() is a macro defined in object.h.  See example 
uses in enumobject.c, genobject.c, and itertobject.c.

DictMixin should add the on_missing() method, leaving the 
responsibility for calling it to core object's __getitem__ 
method.  That will likely never happen, but it does allow 
the mapping API to be emulated as fully as possible.  Add 
the method right at the very end (after __len__).