code: def run_tenuous(self):
          
          with LiveExecution.lock:
              ns_snapshot = copy.copy(self.ns)
              try:
                  source = self.edited_source
                  self.edited_source = None
                  self.do_exec(source, ns_snapshot)
                  self.known_good = source
                  self.call_good_cb()
                  return True, None
              except Exception as ex:
                  tb = traceback.format_exc()
                  self.call_bad_cb(tb)
                  self.ns.clear()
                  self.ns.update(ns_snapshot)
                  return False, ex
docs: Run edited source, if no exceptions occur then it
          graduates to known good.