Traceback or call chain

We now have since quite a time the current subroutine and the current 
continuation in the interpreter context structure. With that at hand, we 
should now be able to generate function tracebacks in error case and we 
need the call chain too, to optimize register frame recycling.

Whenever a continuation is created, we have to walk up the call chain 
and mark all return continuations as non-recyclable.

Should the traceback object be avaiable as a PMC?
What information should be included in the traceback (object)?

Comments welcome,
leo