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I agree with Armin that this could easily be a never ending story.  Perhaps 
it would suffice to sprinkle Py_EnterRecursiveCall around as we find holes.

It might have to, because I can't really think of a better way of doing this.  
The only other approach I know is that of SBCL (a Common Lisp 
implementation): it mprotects a page at the end of the stack and installs a 
SIGSEGV handler (and uses sigaltstack) that knows how to abort the 
current lisp operation.  Somehow, I don't think we want to go down this 
line.

Anybody have any other ideas?