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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _unregister(self, cls):
 """Unregisters the given class. If the specified class isn't registered, ``NotRegistered`` will be raised. """

        if not issubclass(cls, self._registerable_class):
            raise ValueError('Only %s classes or subclasses may be '
                             'unregistered.' % self._registerable_class)

        if cls not in self._registry:
            raise NotRegistered('%s is not registered' % cls)

        del self._registry[cls]

        return True