As Amaury said, lexically nested exception handlers make re-raising
behaviour buggy. In Py3k, a workaround is to instead write:

   try:
      raise Exception("foo")
   except Exception as :
      try: raise KeyError("caught")
      except KeyError: pass
      raise e

With the slight inconvenience that the "raise e" line will be appended
to the original traceback.

If we want bare "raise" statements to work as expected after a nested
exception handler, we'll need to add proper exception stacking, for
example by adding the exception value as a member of PyTryBlock. The
effect on performance should also be measured.