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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 dec(self, key, delta=1):
 """Decrements the value of a key by `delta`. If the key does not yet exist it is initialized with `-delta`. For supporting caches this is an atomic operation. :param key: the key to increment. :param delta: the delta to subtract. """

        self.set(key, (self.get(key) or 0) - delta)