<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def dead(self):
 """Whether the callback no longer exists. If the callback is maintained via a weak reference, and that weak reference has been collected, this will be true instead of false. """

        if not self._weak:
            return False
        cb = self._callback()
        if cb is None:
            return True
        return False